Wait what are shoulder screws?
This is a shoulder screw, and I don’t think it would help make your tray smoother.
Don’t countersink holes on your tray. Aluminum is thin enough and will rip right out once you put force on those screws.
If you use c channels instead of angles you can hid almost all of your screwheads, you just need polycarb for the back of the tray.
I dont find that to be necessarily true. My tray is made from c channel with lexan on the back and there’s quite a bit of screws studding the inside.
Thank goodness vex gave the cubes chamfered edges