rebuilding seat bottom....need advice

The seat bottom of my original Deluxe interior seat (center section) is rotten to the extent that I'm afraid if I don't replace the wood in it now, there won't be enough to make a pattern.

Tonight I was able to remove the skin and foam without damaging them so I'll be reusing them. As far as the wood goes, it's 1/2 inch plywood. Looks like just plain old plywood. Is the recommendation to build it back in plain plywood and then paint it with epoxy or some other waterproofing barrier? If so, what kind of epoxy/paint etc? I don't want to overcomplicate it since this isn't structural. If I can get 22 years out of my newly remade seat bottom, then I'll be satisfied so I'm not too interested in a composite or anything more expensive than plywood.

Also, part of the foam was glued to the seat base. Was the purpose of glueing this simply to hold the foam in place while the skin was put on, or was it put on to keep it from shifting after the skin was put on or both? What kind of glue would you use to reglue it to the new seat bottom prior to reinstalling the original skin? Also, there was plastic on the foam too prior to the skin. How necessary/important is this?