Disposable water bottles are a tricky substance to work with, apparently: PET plastics do not glue, or heat-weld easily. However, people have been using them to create bicycle fenders.
I assume that other styles of aluminum and pop-bottle fenders might be assembled out of bottle plastic “scales” and zip ties, also.
Bent PVC pipe. This is actually really cool, but I read that once bent with a heat gun, it becomes pretty brittle. Love bending it around a rim, tho, totally appropriate tool to use as a form.
Bamboo is pretty expensive, commercially, and needs to be laminated or varnished.