Yeah, I initially used 1/4" CBs for my joysticks, but the Happ hole isn't quite 0.25", so while 2 or 3 of the bolts will fit, you're going to end up threading the 4th one through the plastic, and good luck if you have a metal joystick top. My local True Value sells 3/16" carriage bolts, but they weren't in the same section as the rest of the carriage bolts, so I had to look.
Otherwise there's always Bob Roberts.