Lots of Choices
"Store-bought" grips with internal switches and buttons can be bought from sources like Ray Allen or Infinity. The former offers a single button PTT on the low end and the latter offers enough buttons on their sticks to just about run the entire aircraft-A/P disconnect, 2 or 3 axis trim, Xponder ident, etc, etc. For the passenger stick, many builders do just what you suggest - use soft foam bike grips with a plastic cap from ACO. Many prefer the pax stick to be easily removable, using quick disconnect pins to lock and unlock it in place. In these cases, it's good to have a momentary (PTT) switch mounted on the IP for passenger use. These switches are available at AC Spruce and most electronic stores.
Terry, CFI
RV-9A N323TP