I don't know much about the back mounts for 4x5 cameras yet, but if I couldn't just buy the back I wanted somewhere then I'd just buy something like the osciliscope camera back that I just bought, or a really cheap packfilm polaroid with poor plastic optics, etc, that I won't feel guilty chopping up, then chop up the back/camera as needed and graft it onto something that can clip to whatever camera I have.

I don't know how well it works as a focusing glass but I recently took an expended packfilm cartridge, took it apart, and taped some tracing paper over the window where the film would be. Then I can just insert it into a camera, leave the back open, and hopefully use it to focus. So even if you just chopped up and duct taped a polaroid holder to a camera, you might be able to open up the back, insert the front frame from an empty pack film cartridge covered with something frosty-like, and not have to take the polaroid back off the thing.