Another option would be the one-use polyethylene pots plants are sold in at garden centers. They also have the desired
built in curve, are thicker than the bottles mentioned and designed to last years in full sun. Generally available in black
only however. The "1 gallon" size is about right, but "2 gallon" will do in a pinch. One has been on the end of a too
short fender on my bent for 15 years.