Here's another project that would work with recycled billboards. Many leather projects will also work in recycled billboards. If the particular billboard is not thick enough, perhaps two layers glued together would be stronger.
I work with recycled billboard vinyl advertising tarps. Those products are for sale available at http://www.vinally.ca/ I also sell at http://www.etsy.com/shop/VinAlly
I live in Calgary Alberta. I worked for 12 years in the oilpatch as a geologist/database analyst. I also have a Certificate in Visual Design from the University of Calgary. I also paint and draw cartoons on the iPad. Vinally2010 on Flickr.