I like it, lite, yet robust, clean lines. I´m impressed.
I´d cut a piece of waterproof plywood, to fit in the bottom, cut to contact as much of the base as possible. Distibute the wieght, more evenly and probably double the overall strength, with just a little more wieght. You cold probably, bolt some tie downs, on the plywood so you wouldn´t, stress (bow the plastic pipe). A couple of handfulls of 3/16 " bolts, 3/4" washers and cap nuts (preferably in stainless). Cut a curve in a peice of 2X4 to match the curve of the pipe, put a washer between the two pieces of wood, wack it with a hammer. The washers will fit the curve of the pipe and look a lot cleaner.
Just a couple of thoughts. I really like how your project turned out.
