I used 1.5x1.5x1/8 angle and 3/8 treated plywood all the way around. Given the size of the boxes that will hold anything I can fit in there. Holding up fine, I wondered about the plywood then figured every race car trailer I ever owned had plywood floors did just fine. At least I figure they'll outlast how long I plan to have the truck. I just ran a bead of sealer around everything while I was screwing it down. Make sure everything is water tight, you will be amazed at how much water gets in through something as small as a screwhole if it is inline with the tire spray. Ask me how I know. lol.
There a bunch of RV surplus dealers in White Pigeon, MI and Elkhart, IN. worth a trip before you get too deep, many deals to be had on surplus, plus they have all the "right" stuff to make the job look right.