What material can I use for a seat base that will not rot?

Starboard, heavy though or use epoxy resin to encapsulate plywood

pvc board

You're in Michigan, try ALRO Metals/Plastics. They have HDPE/LDPE sheet usable drops in different thicknesses. The one in Ann Arbor has more than the one in Jackson.

http://www.alro.com/locations/location.aspx?loc=AA

If you decide to special order, the main/corporate building in Jackson off the freeway will be the spot.
http://www.alro.com/divplastics/plas...yethylene.aspx

http://www.alro.com/divplastics/plas...t_pvcfoam.aspx

I have not had a good experience with just resin coating wood, it cracks and also you break the seal when stapling through it.

Yep. This is what Velocity uses. Well, used to use.....

I would contact Tyson (OSO " BigSilverCat" ) he can make you some super nice billet ones. You can also contact his wife Tristan ( OSO " TriPaC87" )

The problem with PVC is that it has poor structural strength. If the seat base is supported well from underneath then it'll be fine, if not move on, it will also deform from gravity if left unsupported long enough. I have no idea what the specific structure is for the seat you're working on, maybe some more info would help. I work that that garbage all day sometimes if we're trimming out houses.

starboard

We just completely redid my rear bench seat. We used 1/2" PVC. Then for the seat bottom that needed additional strength, we completely laminated the PVC with fiberglass and even placed some wood reinforcing "ribs" underneath the bottom, between the metal supports. That was the only wood used, and it was completely glassed on. Turned out nice and strong, and will hopefully last!

Good luck,

JCG