There are a bunch of aluminium Lily58 cases out there, they're not cheap tho.
Most of the time keys popping out means your plate is too thick or the switch holes aren't a good fit. You can often improve it (and the sound) with some foam between the plate and the pcb - you can buy laser cut neoprene plate foam for lily58s, or you can always freehand it with a knife and a sheet of 2-3mm foam (it's not that hard and doesn't have to be perfect).
Most of the time keys popping out means your plate is too thick or the switch holes aren't a good fit. You can often improve it (and the sound) with some foam between the plate and the pcb - you can buy laser cut neoprene plate foam for lily58s, or you can always freehand it with a knife and a sheet of 2-3mm foam (it's not that hard and doesn't have to be perfect).