As far as resin casting goes, you can buy keycap molds pretty cheaply, or build a mold-casting box out of LEGO. Then you can use whatever keycaps you already have to make silicone molds.

They decided to compare the aluminum molds they usually use with some 3D printed molds created with a resin printer. They used two different resins, one on each side of the mold. You can see a ...