Patrick - I started with the LPL 35mm glass carrier from KHB (which only has a top glass), asked them to mill out the bottom opening to the same size as the top, and include some spare top glass mounting tabs and screws. This was done for a nominal fee. The supplied tabs and screws make installing the top glass very easy.

For the bottom, I drilled two holes on each side of the opening (you could easily have KHB do the drilling for you) in the bottom plate and used some brass plate stock from the local hobby shop (you could use aluminum or whatever else as long as it is rigid without flex) to make some supports for the bottom glass. These supports are below the bottom plate (screwed in). The bottom glass sits on those. Some careful shimming is required (again I used brass stock from the hobby shop, but extremely thin sheets) to ensure the top of the bottom glass is perfectly flush with the surface of the bottom plate of the negative carrier. Some trial and error is involved there but it is not difficult.

For both the top and bottom glass I used the cut down B+W MRC filter glass from Schneider (2mm thick). Note in retrospect using that fancy glass for the top glass is more or less superfluous and a waste of money. You'd be better off just getting some plain or anti-Netwon ring glass from Focal Point (or even using the glass that comes with the LPL carrier). A variety of options work for the top glass.

This probably reads more complicated than it actually is. I'm not a machinist, and this was a fairly simple project. Mine ended up being more complicated only because I made it with pin registration functionality for masking, but even that was not very difficult - it just required some more planning and thought before getting started.

It ain't pretty looking (prototypes usually aren't, although building a second one would be much prettier), but it is precise and works perfectly.