[rcg66]

I used 2 sheets of regular 3/4" tongue & groove plywood in my longbed. I primered it, and then sprayed it with rattle can bedliner.
I relocated the gas tank in the bed, and found that regular exterior paint didn't like any gas splashing on it.
It has held up really well for over 5 years now.
I ran a bead of silicone down the seam in the middle just for peace of mind.
I don't drive it a lot in the weather, but I have hauled stuff in it and it is still in really good shape.
I do have the gap at the back that I didn't fill in with anything.