What about the very, very cheap piano pedals? Those are usually just a on-off switch, which is output over a 6.3mm jack. Connect that to an Arduino or something which emulates a keyboard and Bob’s your uncle.

Usually, logic levels are not a big issue, just saying. You have 5v anyways, all you need for a unidirectional level shifter for low baud rates is a mosfet and a resistor.