dc3232, and Baitcaster, sorry I was cleaning up the links to thread and accidentally deleted your first post, I was trying to move it to its own topic. Anyhow sorry for that.. Post away.. As was said before i screwed up. A lot of guys are using the blue/pink foam from lowes and home depot and such.
I stacked foam boards (2 boards glued with gorilla glue and sprayed with urethane) 2 high and it made them even with the ribs. I then put plywood on top for the floor. Its awesome works fine. and supports the plywood floor great.https://www.tinboats.net/forum/viewtopic.php?f=21&t=17299
You will want to use the pink "closed Cell" foam. I used regular Styrofoam that said it was for exterior and interior use and was moisture resistant. I coated it with urethane. But I plan to replace it with the closed cell at the first sign of water logging.