The purge valve is pretty basic- if you took a flat piece of rigid material, and wanted to add a one way valve, do the following-
Draw a circle
Bisect it with a horizontal and a vertical line
Cut out within the boundaries of the quarter sections that you just drew, leaving the cross lines with enough material to retain rigidity, and leave a center circle in the middle
Drill a hole in the middle
Cut a chunk of rubber (truck innertube?) in a circle slightly larger than your cut out quarters circle.
Use some kind of rivet in the center of the rubber circle and the material circle.
For example, you could use something like this in the bottom of a plastic 5 gallon bucket, for a "self-filling bucket". You push the bucket down into water, and the flap comes up, allowing water to fill the bucket. You lift the bucket out of the water, and the flap closes, keeping the water in the bucket from pouring out the valve.
Just remember, the "holes" need to be small enough that the rubber is not pushed through them by the force of whatever it is holding. You may need to leave more cross bracing to keep this from happening. And because of this, numerous small valves may be better than one large one
gomi_