One size fits all footpaws?

Hi all!

I was wondering if anybody had a tutorial, diagram or even the slightest idea on how to make a footpaw that you would be able to use your own sneaker for, instead of having to build on a shoe? I like the idea of having a pair of footpaws that my customer can put whatever shoe they'd like into the paw, but I haven't had any clue on how to make such a thing!

Thanks for any advice! :3