USB pedals or foot switches are not expensive, but nothing beats free. Redditor shrodingers_Cat took out all but four of the keys on his spare keyboard and then taped DVD covers over each of the remaining keys. Voila!
The hardest part of this hack is cutting the DVD covers to fit the keyboard. Shrodingers_Cat says you might want to test firmer sheets to step on, but he’s been using his ghetto pedals to play Elite: Dangerous and he says they’re holding up.
Alternatively you could just put a keyboard as is under your feet. Give your toes some much needed exercise.