A couple thoughts:
1. Your hand when resting naturally over the buttons shouldn't be straightened out. You don't want to be finger slapping the buttons, you want to be pressing them with the tips. On my first hitbox I made it so they were under my finger tips when they were extended, this was not good. Playing for an hour or so was no big deal, but there are some days where you want to play for longer (for me it turns into whole days) with the fingers straight out, my knuckles and joints hurt like hell after a lot of playing. I re-made my top plate so that my fingers rest over the buttons curved so the finger tip is facing more down at the button, this is much better.
2. Your T-molding will go on there much easier if the corners are rounded.
That thing looks like a beast though. Good job so far, I can't wait to see it finished.