Try Griots Garage. They have a great product that you cut to your own dimensions. I've installed them on the Cobra Valley Tri-Lite housings, and it worked perfectly. You can't even tell they are there, but the product is thick enough to protect the lense. You just have to cut them to the dimension of the lense. I don't think it will work on a domed lense very well.
http://www.griotsgarage.com/catalog.jsp?&SKU=10227