Find a cylindrical momentary switch and mount it in a piece of tube, or a piece of wood that you drilled a small hole through for the wires and a wider, but not through hole in for the switch.

You also might want to look at shops selling adaptive tech such as https://enablingdevices.com, which has, for example https://enablingdevices.com/product/thumb-switch/ or https://enablingdevices.com/product/pinch-switch/