From the looks of it, this should make 3D printing your own mockup pretty easy, too. As a springboard, Valve has dropped a few variations for the controller's battery door (below) that have a cavity for holding the wireless USB receiver.

When we last wrote about the Steam controller at length, we mentioned its low-hanging face buttons and sharp plastic seams as design quirks -- maybe what you come up with will feel a little better in that respect. Given Valve's generally good graces regarding profit splitting with its community creations, it doesn't seem like Gabe Newell and Co. would have much of an issue with you selling any accessories for the gamepad.