If the back is flat, I would add a thin wood backing strip of hickory, hard maple, ash, depending on how much you want to increase the weight. If you are working with a crown or rounded back, your options are somewhat limited. You might try rawhide, but I think there is a debate about how much, if any, draw weight it adds, though. Other backing choices are linen, silk, paper, etc., but not sure they add much draw weight.