You will need:
1) a junked USB computer keyboard
2) something foot-switch'ish– a musician's stomp effect pedal would be ideal.
Rip the keyboard apart, and determine which of the membrane contacts are the row and column pins for the F5 (..or whatever key you want). In my case they were covered in carbon, but that scraped off and the trace underneath could be tinned without any problems. Wire those up. Wire to switch. Live the life.