Ass Savers: An autumnal discussion

These fun, no frills fenders at http://ass-savers.com/ seem like a cool idea and I really like the nifty video, but how hard would they be to make at home? 

Anybody ever made anything like this?  Did it work/not work?  Stashing one of these under a saddle or in a bag seems like a handy alternative for those days when you dont happen to have your clip on fender and its pissing down cats and dogs..