My dentist of a few years ago (specialized in cosmetic dentistry and pushed gold every chance he could) made me a night-time bite guard of transparent plastic that probably has saved me from replacing several fillings. He first took a cast of my lower teeth in some kind of plastic/wax substance, cured it, and had a plaster cast made from it. The plastic bite guard was made from the plaster cast. Most of this was covered by insurance, although I have no idea how it was justified. If your retainer is more of a positioning thing, see if you can get this type of guard made. (The sports guards at discount stores are too bulky and prone to pop out of your mouth; the one I have has only come out once in about 5 years.)