How timely a discussion-I'm there right now also!

I agree with your choice of POR-15-doing that myself. I plan to also coat the underside (road side) of my floors with it too, then cover with rubberized undercoat to protect the POR from rock chips.

Your suggestion of the roofing material sounds interesting-I'll have to look into it. But I think it might be prudent to provide a few drain holes in the floor, just as the factory did. The floors on my car rusted out because of leaky window seals which allowed the water to enter, and from the short-sightedness of the person who applied the original undercoating-they plugged all the drain holes so the water had no chance to escape.

I think I'll avoid putting anything in direct contact with the floor that could absorb & hold moisture.