I shimmed out my fork clamps (adhesive-backed sheet rubber,
https://www.amazon.com/33-P007-062-002-036-General-Durometer-Adhesive-Thickness/dp/B00P5VQ7HE/ref=sr_1_5?ie=UTF8&qid=1508024846&sr=8-5&keywords=adhesive+rubber+sheet ). I put the adhesive side against the inside of the clamp, so I can move it wherever on the fork tube it wants/needs to go to position the bracket correctly. They now don't move on the fork tube at all.
It helped quite a bit. The for/aft movement is completely gone, but the shield still does rattle side-to-side a lot more than I'd like...it bangs against the mirror stalks almost constantly as I ride, the left side more than the right. This is clearly caused by the flat brackets not having enough rigidity in that plane.
Other than that, though, the shield is quite nice. It's much tighter to the front of the bike than my old Slipstreamr was, which is definitely a good thing. It looks much better on the bike and I don't get a huge reflection off of my headlight, since the shield is behind the headlight. I don't know if I got the smaller/lower vent that gyrocyclist was talking about, but I don't really notice it much at all. There's very little air movement behind the shield at freeway speeds. It doesn't cover my hands like the Slipstreamr did, but since it's mounted further back that would require pretty major changes to the shape.
I got the x-wide, x-large shield, which is probably why it hits the mirror stalks. The narrower ones might not do that. I'm not that tall, 5'11", though I do have fairly short legs (28-29" inseam), but I was surprised that I can easily see over the top of the x-tall screen, line of sight of the top of the screen being 10-15 feet ahead of me on the street. It's perfect for me, really. There's a lot less wind hitting the top of my helmet than with the Slipstreamr, though it mounts a lot closer to me (or maybe BECAUSE it mounts closer).
I've definitely decided to use it rather than the Slipstreamr. I'll probably pad the mirror stalks so they don't chip the plastic, and I'm toying with cross-bracing the brackets somehow so they don't rattle from side to side. I'll also be ordering some allen-head stainless steel attachment hardware, which I've been doing all over the bike as I work on it. Other than that, I'm a believer now. Earlier this week I wasn't happy I'd spent my money on this windshield, now I definitely am.