This is a great idea!
I've been saving all my old bottles, making rafts and using them to waterproof the outhouse roof, and as junctions between the gutters and rainbarrels.

Have you tried this your self?

Not yet! I've just got back from two months away and the state the boys let the house get into I'll be needing shovels and brooms rather than little hand brushes for a few days...

I'll post a report when I've cleared out a bit and had time to try making one.

do they biodegrade in the sun? I've heard a lot of people using them for roofing, but I'm concerned about them biodegrading like milk cartons do