What about this cap?
Item: http://www.digikey.com/product-detail/en/TAGRED/EG1095-ND/101349
Spec Sheet: http://spec_sheets.e-switch.com/specs/T041604.pdf
I think this will fit the post of the hat--may need some glue to stay--but will it be long enough to stick out of the handle to actuate the switch and will it fit through the hole in the handle?