I've modified a spare hatch cover by inserting a car valve in the middle of it. Simply cut the valve out of the tube leaving some excess rubber around the base, drill a neat hole in the cover and push the valve in from the underside. It's a good idea to fit a second valve with the valve mechanism removed and attach a balloon to it. This will insure you don't over pressurise the hull which can cause catastrophic problems and it doesn't take much pressure to do damage so be careful.
