I think I got it...
It took some snooping around but my wife found a sleeping pad at Kmart. It's a closed cell foam pad to go under your sleeping bag. It's about 3/8" thick and looks like it'll do the trick.
I tried making a copy of the original gasket (this is a disc of this foam that sticks to the vent door). That didn't seem to work, maybe the foam was a little to thick or something.
So, I saw the impression of the vent opening on the back side of the foam from when I closed the vent. I trimmed a circle out of the foam and stuck it to the outside perimeter of the vent hole coming from the trailer.
This seems to work fine. It is adhered to the trailer with spray adhesive and the vent cover holds it in place when closed. This will create a lip around the opening that will keep the water from coming in.
Tomorrow when the glue is dry I intend to do the watering can test to make sure. Initial impressions are that it will work well.
1993 34' Excella 1000
2005 Dodge Ram 3500 Diesel
(No it doesn't have a HEMI in it)