Rebuilding wood bed on C30, pressure treated lumber and rust

[unbrokenchain]

So I'm going to rebuild the bed on my C30 because there are very few solid oak boards left (which is a bit of a shame, I bet it was beautiful at one point). Conveniently I am also disassembling a large deck on my house and will have plenty of planks in good shape for the bed surface.

The carpentry is straightforward, no issues with the building itself, my question is about the load-bearing wood that rests on the frame. It seems like the salts used in typical pressure-treated lumber would accelerate rust/frame rot. I'm planning to wire brush and coat the frame with POR or Rustoleum, but is there something additional I could put between the PT wood and the frame itself? I was thinking maybe some type of roofing tar/flashing or even laying down a strip of tri-flex. Anybody have any ideas, or does it even matter?