...the painted panels full of circular holes that make up most of the horizontal parts of the trailer look like standard "perforated panel" pieces from vents or light panels in a dropped ceiling, or something like that. (Might even be metal instead of plastic -- it looks like metal if you look really closely at the cutout around the cylinder beside the red car on the lower deck.) Both look like a really clever re-purposing of something from the hardware store!
Looking at the joints between the beams, they almost look like they might be welded; the whole thing could be made out of the rectangular thinwall metal tubes that cheap furniture is sometimes made out of, or small aluminum bar stock, or something like that. Or it might just be plastic, but the way the cylinders are screwed into it looks like it would have to be stronger than most plastic.
The hydraulic cylinders are two sizes of aluminum tubing that slide into each other; if you look closely you can see how they're flattened at the ends to make the attachment points to the frame.