Done a few cases on 3d printer.
Right off the start you have an issue you need to deal with and that is the plate, 3mm might might work across such a small area but it's easy to add a few reinforcement ribs underneath. They don't need to go into the clamping area but they should be under the bulk of it. Should be a simple, easy fix. Be warned, while this means you can easily just print the plate upside down, and you can, if your printer has any skew // or \\ it will be opposite of the rest that were printed right side up. Rotating it 90 degrees before you print should reverse the skew, I'd also give yourself a bit of extra tolerance here. Again with your keyboard size it's less an issue compared to something bigger but it is something many don't realize can be a problem until they encounter it. Pretty much every printer has some skew, particularly deltas and corexy, deltas can be all sorts of weird.
As for attaching it,
Make your entire top section one part, not one section recessed into the other. Not only is this unlikely to create the flush smooth top surface you expect because of how printing works (you're likely to get a low corner) but this gives you more area around the perimeter. With this wider perimeter you can extend your support ribs but also this gives you meat to simply drive screws up from the bottom. Some will say to use brass inserts in the top and some nice bolts from below. Meh. It's plastic, cheap and easily redone. Use machine or wood screws, on mine I used 3/8in #4 screws (I think). Before assembly, create threads by screwing it into the part until it bites, then heat the screw with a lighter (blue flame, not yellow) until you see it lean or just use your gut. Then drive it in and let it cool. Once you do them all pull them out and assemble the keyboard. This has worked great for me on bunches of projects.
Plastics...
PLA will work fine here but has a higher pitch tone and if you leave it in the car will deform. PLA also deforms over time when under pressure,your switches will eventually feel lose as will the screws as any pressure points will slowly lose grip. ABS will work well and not deform but ABS is a hassle, and while strong when done correct, if it has a problem it will have very little. It's sound is a bit lower. Polycarbonate is not recommended since you will be touching it constantly (bad chems).
My advice is PETG, it's sound is lower than the rest, will not deform, resists stains and resists warping from heat almost as well as ABS. It's also cheap. The downside is you can't glue it as little sticks to it (paint can also be a problem) and fewer color options but it's the better choice for use. I was quite surprised by the sound of mine in Petg, it did not sound plastic, well, I mean, it did, but not the common cheap plastic sound you expect. It's much closer to aluminum in terms of pitch than the others.
If you outsource this they will likely use little infill, probably 3 perimeters, 10--25% infill. Not only is it small enough but the cost difference is negligible for you use to use a lot higher infill. I would highly recommend 40-50%. Not just for strength but also sound and weight.
There's a wealth of info in my thread
here and on the Thingiverse
page for it.
