Ok, for the purpose of completion- here is my finished trailer box. Not the prettiest thing in the world, and probably not the most functional (a wee bit small) but heck, it keeps the sails out of the front seat when driving the forrester.
I used cardboard concrete molds (the kinds you put in the ground then fill with cement) cut down the middle and then stretchd apart about 12 inches. The front of the box was done first then I bought 2 more and put those together. As you can tell, evidently the quality control isn't very good because the circumference of the first tube did not match the other 2. Realizing this ain't rocket science I figured I would just join them together anyway. Besides, more aerodynamic
I used 3 layers of glass cloth and painted the inside of the tube. The bottom is a single piece of pine board with a routered slot for the tube to fit into. The board was coated with epoxy. The end caps are just plywood. Total cost was probably around 150-200 (and significant back pain
BTW- the paint? Flat rustoleum... oh YEAH... gigly gigly gigly