small sized waist level finders are useless in my opinion. I have a compact with one, the Canon Sureshot Ace. Its a very neat a cool gimmick but doesnt have much of any use, unless your a fan of shooting lying on the floor, or holding your camera upside down in the air.
You may be able to rip one of those out from these compacts for your use.
All you really need is a mirror set at 45deg. angle. I don't think it even needs to be a first surface mirror, but perhaps that would be better. But then I suppose you want the image corrected(I'm so used to TLR's and view cams). Find a dead SLR and rip the pentaprism from the finder, use 100% silicone caulk to glue it in place. Just a few small dabs will suffice.