Since it's enclosed, I presume you're not hauling dirt/mulch/stone/sand/etc. I have D-Rings recessed in the wood deck of my OPEN trailer, but that's only because I needed "chunky" tie-downs that wouldn't get fouled with all the stuff I'd be carrying.
If I had an enclosed trailer, I'd certainly look at the T-type track systems they sell at TSC and trailer stores. They're quite stout and provide nearly infinite tie-down possibilities, especially if you're liberal with where you put tracks. My brother put these in his enclosed trailer and put another sheet of plywood floor down wherever the track WASN'T, providing a flat loading surface.
He even put some of the tracks on the walls, further increasing the utility of his straps by allowing him to anchor things in odd orientations.
It seemed like a nice solution and works well for his varied needs. He hauls a show car and uses it for random homeowner construction purposes.
