My local HD/Lowes have PT 1x's (3/4" actual) and "5/4" decking boards (1" actual). I would think either could work pretty well for a trailer. I have also picked up composite decking really cheap on a closeout (used it to redo a picnic table) and they often have short lengths of various 4' or so PT in the bargain bin.
I suspect fir won't last too long unless protected, but if you keep the trailer under cover and don't use it for wet things it might be OK.
If you have access to local roughsawn, white oak will last a while. 1x roughsawn (1" actual) could work really well.
My Dad built a trailer out of white oak, a car axle, and a 4' square x 1' tall metal cargo box he scrounged from somewhere. It had stake pockets, so with a stake in each corner if you stacked it right it could hold half a cord. The oak was soaked with used motor oil when the trailer was made, then used the heck out of it for 30 years with no further care. It was stored out in the elements, the most he would do was put a chunk of wood under the box (manual tilt) so the box wouldn't collect water. When he passed the oak frame was just starting to rot to the point where one would think about replacing it.
If you are worried about the dimension of the wood taking up cargo space, inset the wood into your frame. At each pipe, weld on chunks of 1" flat bar with mounting holes, so that when you bolt/screw on the wood, it's flush (or almost) with the pipe. The pipe would then help protect the wood from getting dinged up at the ends, too. If you wanted a tighter fit, you could bevel the wood ends where it buts against the pipe. You could also run the boards in the upper angled section of your frame up and down, instead of the long way, if you thought you didn't have enough support in that direction.
. Still gonna keep my eyes peeled for some cheap sheet metal.
