One tip I got that is something I’m going to do is to silicone the edges of the plywood. So where your plywood is showing in the back there, I’m going to silicone that edge to the van so if there’s ever any water that tried to get in there, the silicone blocks it from getting trapped and getting moldy. Helps keep dirt out too.
Right on, I polyurethaned all my edges and sprayed them with clear Flex Seal. I'm going to run a bead of black seam sealer along the bottom edge to fell the gap between the wood & metal floor too to keep crap out of there.
I used a wall-to-wall layer of Floor Muffler Ultraseal
under the wood as a moisture barrier. It also did a really good job of soundproofing. Gonna use another layer of it between the wood and the carpet because I have enough of it left to cover the whole back of the van and figure it will make the floor even quieter.