This is something that I'm seriously thinking about doing as well (particularly as finding plate holders for my FKD camera is particularly difficult).
I was under the impression that as long as you have a 'U' shaped channel that the darkslide can slide into, it'll be OK (as light cannot bend round corners). So that would be three sides taken care of, the side where you would slide the DS in and out, would have a strip of velvet running across it to stop light entering on that side.
I was thinking about using a wooden frame, but have bent metal over it to make the U channel. It's the only simple way I could come up with to create a relatively tight fit for the dark slide.
Perhaps, I'll put a drawing together to explain that!
Cheers,
David.