[dsraven]

when you get it figured out after a test run you could try using a plastic knife gate or slide gate valve for plumbing. it is used in stuff like hot tubs and spas and such. you could install it wherever and have a cable to operate it. like if it was under the seat, say, you wouldn't have to move the seat to open or close it.
if you want it under the seat that is. they seal the weather out and don't take much space. I will punch a hole in the back wall of my truck because I want it to have flow all by itself as well as the ability to equalize pressure while closing a door etc. to each their own.