I think you will be far happier with the real thing.make one for me too...
I made a 6x12 with a 90mm lens, but its not very discrete, then again, Ive used a Mamiya rz out and about on the street and that ways a ton and is like a canon going off.
Wouldn't be too hard to adapt a medium format to a double frame 35mm, sure theres a DIY youtube vid somewhere.