In addition to the permanent fabric floor (that will be in reality soon), there is a detachable wooden floor made from 1/2 plywood (not pictured). Heavy duty stuff. It's strong, and it's really heavy. The complete wooden floor weighs about as much as the trailer frame by itself.
[By the way, I don't have an exact number but the trailer frame weighs a little more than a common bicycle, probably about 30-35lbs. I could see a reduction in weight if I were to switch to more expensive materials like thin-walled tubing of chromoly or 6061 aluminum. The reduction in weight would cause an increase in cost, no surprise. ]
Anyway, about the detachable wooden floor, it looks like a wooden stretcher, as it is cut to fit exactly into the trailer. The detachable (and heavy) wooden floor would be used for occasional trips in which the trailer needs to carry highly dense or sharp objects. The reason: fabric will tear if you concentrate the load at one spot. For example, if the trailer needs to go to a landscaping job and carry some tools like shovels, rakes and also some heavy bags of mulch and soil. Oh no! Wouldn't want the rake to pierce the fabric floor! :)
A friend of mine joked that I should paint the wooden planks yellow like the rest of the trailer, then it would truly be the cheesy trailer.
The top cross pieces are not fixed in position, they are attached in such a way that they can rotate and be stowed out of the way in the case that the trailer is used to carry something taller than the side walls. Something like a 4x8 sheet of wood or mattress or...[fill in the blank]