Brief update. I've found a couple of things that have helped.
I've changed my timer so that, after it is full, it stops charging for 15 minutes and then starts again. This seems to help with topping off the battery. When I've just left it plugged in, I'd often lose a few percent more than expected in the first couple of miles as if it wasn't actually fully charged.
From your suggestions, I've insulated the battery with neoprene. I had read about this from Brian T Rice years ago, but never wanted to do the arts and crafts necessary to make the coozy. With a bit of time thinking about it, I was able to use an old pair of wetsuit pants as a cover without using any tools unless you count a bungee cord as a tool. I'm sure it would be more effective if I did a better job of making something that fits well, but this has helped significantly. Where before I would sometimes have to go 45 MPH to make it through my 45 mile commute, I can now do the same commute in the same temperature at 55-60 MPH.
Thanks all