ElectricMotorcycleForum.com

https://www.ebay.com/itm/36V-20Ah-Li-ion-Lithium-Battery-Pack-For-Electric-Bike-With-5A-Charger-50A-BMS/163711527429?hash=item261df7aa05:g:Mt4AAOSwNQNc5jmV
Would fit vertically, spanning top and bottom compartments after removing a small amount of material from the battery support channels.

This is definitely interesting.  It appears to be based on pouch-type cells vs. 18650 cells.  I know very little about these.  The weight is surprisingly heavy at 13.2lbs (my custom 20Ah 18650-based battery is around 6lbs (I''ll measure it exactly one of these days).  Still, 13.2 lbs for 20Ah is less than half the stock SLA weight and ride time is probably triple.

https://www.ebay.com/itm/36V-20Ah-LiFePO4-Lithium-Rechargable-Battery-For-500W-Ebike-Scooter-5A-Charger/162323380088?hash=item25cb3a3778:g:xmYAAOSww8ldPqfC
Would fit in top compartment, coming very close to the controller.

This one I'm less excited about but still intrigued.  At 17.2lbs, it's pretty heavy for a lithium battery but being LiFePO4, my understanding is it will last a long time and has a safer chemistry than LiPo.  I converted 2 of our Razor Ground Force Drifter karts to use 24v LiFePO4 batteries I bought used on eBay and they've been absolutely great.  The run time is plenty long to overheat the motor so I have to remind the kids to give their karts a break. :-)

Thanks @MDW195 for the reply

I don't know all the benefits but I did have a good conversation with Marllon @ Kuberg Florida https://kubergflorida.com/ about upgrading the controller and all wiring to the latest version.  In my recollection, it wasn't that expensive (less than $300 I thought).  He's a great resource and handles most US tech inquiries.  It's possible the Kuberg throttle isn't specially made, looks like an e-Bike part to me.

I'm really surprised that your pack fits right between the rails.  Mine are too close to fit an 18650 battery sideways so a whole pack would require the rails to be cut/ground to make room.

Yesterday I reassembled the pack using cell spacers.  I like these way more than the hot glue.  That said, they're a bit of a pain to work with and they do break very easily so always buy extra (maybe 30% more than you need to play it safe).

Awesome, thank you for sharing!  That's a BIG battery! :-)  I'm curious, what is the cluster of wires on top of the battery?  I can't see enough to make it out but there's a lot going on there that I don't have on ours (2015 Cross and 2015 Trial E).

Nice choice on the Cycle Satiator for a charger.  I went with Luna's Advanced 36v charger primarily because it was 1/3 the price.  I like that Grin's Cycle Satiator is multi-purpose.

For a voltage meter, I have a NON-recommendation: https://www.amazon.com/gp/product/B07ST8MQVL
The above meter consistently read 2v low when I tried using it during a battery capacity test.  I've had decent luck with similar gauges so maybe I just got a dud or maybe had it wired up incorrectly (I'll double-check). 

Regarding my pack build, I learned that my friend w/the spot welder thinks he might have been exposed to COVID-19 so the thought of trying to build a battery wearing masks and gloves feels...yikes, not comfortable!

In better news, it appears the circuit board for the emergency charger will arrive today so I'll probably focus on testing that out soon.

Wow! Nice find on a Kuberg-specific pack!!  In my opinion, that pack is overkill and too expensive compared to this one:  https://www.ebay.com/itm/laudation-36V-20ah-1000W-E-bicycle-battery-10S6P-Built-in-Panasonic-NCR-18650BD/254369765764

A 10s6p 20Ah pack weighs under 7lb.  My son's 13.6Ah pack is under 5lbs and has *never* left us stranded; my son runs out of energy way before his bike does (so far).  This pack is over 12 lbs, cost nearly 2x more and has cells of unknown brand.

That said, I'd *love* to hear from someone who owns that T-shaped pack to see how it performs.

I did a bit of research and it appears these are a bit different bikes.  The Moto Tec 500 and 1000 w/lithium battery appear to have a battery like this that slides in/out the rear of the bike: https://davesbigtoysusa.com/mototec-dirt-bike-36v-lithium-battery-pack/

As shown on this site, the capacity is 8Ah (500w) or 10Ah (1000w) https://www.urbanscooters.com/products/mototec-36v-pro-electric-dirt-bike-1000w-lithium

Current draw is adjustable from 10A to 35A on the 1000w version.  A Kuberg needs a higher current battery (seems like 40A minimum).

I think a decent eBay lithium ion pack would be easier to find with the right specs and I've been very happy so far w/the one conversion I've done.

The Mototec's best feature is its looks.  My friend and fellow Sur Ron owner bought one for his daughter.  It has no front suspension, just lookalike suspension forks.  Its battery is indeed lithium but doesn't have enough capacity to be practical.  On a ride with 1 Kuberg Cross on old, tired SLAs, 1 Kuberg Trial E on a 13.6Ah eBay lithium ion pack and 1 Mototec, the Mototec died first.  My friend hauled the little Mototec out of the woods ON his Sur Ron.  Not recommended but he pulled it off!

@MDW195 - The battery pack you ordered has some concerning and inconsistent specs:
Regarding the cell configuration, in some place it indicates 10s7p, in others it's 10s8p. 
The weight at 8.4lb suggests it's 10s8P.
The dimensions seem like they won't fit into the Kuberg's frame.  I found a 10s6p will fit but only with the cells staggered.  Using pre-built cell holders, my 10s6p was too wide.
The peak discharge current is 30A and the *Continuous Charge/Discharge* is higher at 50A.  I'm guessing they got these two mixed up.
If true, 30A seems too low for a bike with a 40A controller. 

The pack I intend to build has these specs:

PANASONIC/SANYO NCR18650GA

Cell Specs:
====================
mAh:   3500
Discharge:   10A
Charge (normal):   1.675
Charge (fast)   ??
Cell weight:   48.0

Pack Specs
====================
Configuration: 10s6p
Pack Capacity (Ah):   21.0
Pack discharge (A):   60
Pack charge (normal): ??   
Pack charge (fast):   ??
Pack charge (not clear if fast or slow):   10.05 ??
Pack weight (g)   2880
Pack weight (lb)   6.34
cell cost (ea)   $4.60 (but I found nearly new salvaged cells for $3.10ea)
cell cost (pack)   $276.00

BMS: https://www.ebay.com/itm/10S-36V-15A-60A-Li-ion-Lipo-Lithium-Battery-Protection-Board-BMS-Balance-ebike/322539980221
(waterproof 60A version)

Charger:  https://lunacycle.com/luna-charger-36v-advanced-210w-ebike-charger/

I was planning to build the pack with a friend that has a spot welder but got too busy and now the COVID-19 issue is discouraging us from meeting up to do so.  I might just borrow his welder and see if I can built it myself.  The old, tired lead-acid cells are becoming a real pain and limit our riding a lot.