[@mmc_jake]

This idea has made me think of an alternate approach as well (likely outside the Makey-Makey Contest). My son has a toy that consists of magnetic letters that are placed into a holder. When the letter is placed into the holder, it activates an action and plays a sound.

The letters have little ridges which activate little switches in the holder to trigger the right sound. It would certainly be possible to create a modified set of the symbols that have similar coding on them and a suitable holder to detect and play a corresponding sound.

(Image from https://www.target.com/p/leapfrog-tad-s-fridge-phonics/-/A-52686781)