I had the same thought with my feed bags. I had built 3 chicken coops that were 3 feet by 8feet long to go in my 10x80 chicken runs. The bags were the prfect length to use for the roof. I had heavy duty tarps that I folded over like 3 times and used, but if I build more coops which I will that is what I am doing. I am going to turn them inside out so they are all white and make them 3 or 4 deep on the roof so I know for sure it will not leak. That's just an idea. If you got any good ones to do with them let me know. I thought it might look bad, but when I turned them inside out and put them on the top it didn't look bad at all. So next time I am going to try that. I have also thought of using them to cover the slats in the walls to keep wind out. I didn't do that because I wanted fresh air to be able to come in somewhat.
Yours sounds more like a good working idea. Mine will be more of a crafty kind of thing not utilitarian at all. I was thinking of making some cute coin purses or bags. It will all depend on my sewing skills.