The contoured keywells in the Kinesis contoured and Maltron keyboards are made from plastic, probably ABS.
The key switches are mounted to them, with a flexble PCB/individual wires on the inside.
Suka on this board has made several custom keyboards with 3D-printed contoured keywells, but I am not sure which type of material that he used.
I think that the trick is to make the plate thicker than spec, except precisely where the switch's tabs are.
Also, make perfect square mounting holes 14×14 mm, and make sure that the parts are printed solid.