P3D B&W slide machine

[Nick Merz <merz@xxxxxxxxx>]

I do lots of 3D shooting with a Viewmaster camera and a Sputnik 
medium format camera.  I am keen to do some work in black and white, 
but have encountered nothing but trouble so far.  Attempting to get a 
lab to use E-6 processing on some C-41 black and white film destroyed 
a month and a half of work.  The other option would be to have a lab 
duplicate things at $2 a frame (no).  However, I can imagine building 
something that might do the trick...

The concept is to shoot a roll of regular B&W negative film, and then 
duplicate it on a fresh roll to get positives.  This would allow me 
to do all the processing myself.  The key then is to build some kind 
of a light box that allows me to sandwich the exposed and unexposed 
rolls together and expose them.

I could imagine a disposable camera with slots in the front, to the 
left and right of the lens that were the width of a filmstrip.  I 
would then build some sort of right-angle plastic light shrouds over 
these slots which would then aim them out to the sides, then back 
towards the back of the camera.  This would, in theory, create a 
light-tight path for the exposed roll to enter and exit the camera. 
I would then put tissue over the lens to act as a diffuser, and 
permanently mount some controlled light source, maybe a white led 
with reflector or some of-the-shelf flashlight, aimed at the lens. 
Loading the film, the unexposed roll would go inside, as normal, but 
the exposed roll would be loaded at the same time through the other 
path.  They should sandwich together at the focal plane and separate 
again, new one going to the take-up reel and original coming back 
out.  The chore than is to click and advance all the way through the 
film at the right exposure.

This should be feasible with a cheap disposable camera for 35mm and 
maybe a Holga for the 120 film.  I would almost certainly have to put 
tape-tails of some kind on my original film to retrieve it during 
rewinding.  I can also imagine wasting some film getting the exposure 
right.  I could see a test where I shoot an entire roll of exactly 
the same image at the same exposure, than use that as my original to 
bracket exposures in my light box.  I could also see some problems 
getting things to line up correctly, particularly for the Viewmaster 
rolls.

Has anyone created something like this before?
Can anyone see immediate flaws in the plan?
Any suggestions about exposure, advancing, alignment, light source?
How about a continuous feed system, motor driven...

Nick Merz