Jerry Reshaw made one out of PVC pipe and pipe insulation. 2 pieces cut to length you want, two 90 degree elbows and another PVC piece cut to your width. Cover this with pipe insulation. I added legs to it. Works great. If you don't glue it together, it breaks down for easy traveling. Email me and I'll send you a photo.
Another idea that works - emergency rooms have a foam "carriage" that they lay a broken arm onto prior to putting on a cast. These work great. Now you'll have to find someone that works in an ER.