I am using the AMERICAN HEART ASSOCIATION red rubber band with the original front and back clear protector, still waiting for my bumper case and BSE full body, it is fully functional but I still need to cut out the bottom port and the mic on top.
That's good, I assume it wasn't made as a bumper as such, but a worthy cause too.
Suggest you use a hole punch to cut the holes else they will tear. Only punch the holes you really need, covering the other orifices (headphones, multiconnector) will help keep dirt & moisture out.