How about buying one of those "metal" backs from ebay ? You can get them in all sorts of colors. I got mine for 8$ shipped from a Hong Kong seller. the only drawback is they do not usually have flash diffusers which may cause an issue.
There isn't anything (that I'm aware of) that will work with your bumper. Only options you have is getting one of those metal backplates or one of those carbon skins for the back of the phone. There is this but I'm doubting the plastic shield would work with your bumper case.