It used to be possible to buy keyswitches with plain white flat keycaps which had snap-on clear covers, the idea being that you could print your key legends onto plain or transparent paper, cut them out to the exact size of the keytop, place it on the keytop and then snap the clear cover on over the top.
There's a video here
https://www.youtube.com/watch?v=Ed3zSPnodhE
of an MK14 with an added on keypad using that type of switch, although the ones used in that example are perhaps over-scale. It also illustrates an inherent problem, that the originally clear plastic covers tend to go yellow over time.
It may still be possible to buy smaller switches and keytops which use the same principle.