Great design!
One comment, it looks like your plate mounting positions will result in a very rigid plate feel, which could defeat the purpose of top mount unless you go with brass (which is super rigid anyway) or something like PC or PBT (which is flexy no matter how you mount it).
I'm specifically looking at the mounting tab directly between the spacebar switch cutouts, and the two tabs very close together in the bottom right corner, although those are probably OK because they're under the arrow keys.
The way I see it, a plate can flex in two principal directions: "hotdog" (the front and back fold up to meet each other) and "hamburger" (the left and right fold up to meet each other). Since plates are generally mounted on the front and back, you generally don't have any "hamburger" flex, and any that arises is probably undesirable edge flex, unless the plate is a really flexible material like polycarbonate. So what you have left is "hotdog" flex, which in theory should be pretty consistent across the row, especially on the home row. My fear is that adding tabs on the sides of the plate (instead of just front/back) will impede this "hotdog" flex and defeat the purpose of top-mounting. OIf course, any flex will be less accentuated in a 40% than in a bigger keyboard (and again it probably doesn't matter at all with brass).
On the flipside, having mounting tabs on the edge could reduce resonant metallic sounds from the plate by preventing it from vibrating as freely.
Obviously this is quite a bit of speculation, but IMO it's worth thinking about. Hopefully there will be a prototype made (maybe 3d printed + cheap alu plate?) so this can be refined before the GB.
Edit: w/ respect to the options in IC poll, I would much, much prefer a "budget" all-aluminum, no-weight design. If the Brutal60 can be $200, this can be $150 +extra for brass plate and +extra for fancy case material like polycarbonate or all-brass. I really don't like the value of being premium for premium's sake. If the brass weight really adds something to the design, then include it. If not, then it's just as much of an afterthought as some kind of half-baked gasket mount. FR4 is also a very nice cheap plate option; you can still fancy it up by putting (lead-free) plating around the switch cutouts like in the No1.
Also a half-plate option could be very interesting here.
