I recently built a set of simple holders for paper negatives and calotypes, and one that holds a ground glass. These are one-sided single channel holders, so they are not fancy but they work and they were simple to construct out of 1/8" MDF. The holders are designed for either 8x10 or 7x11 paper negatives. They follow the ideas found here:
http://www.f295.org/main/showthread.php?12458-making-film-holder-need-idea
-- unfortunately the photos at that link no longer exist, but if you read the descriptions you can work it out. Like Mark's, mine are for use in a lensed camera. The trick is to make the distance to the GG or the surface of the paper be determined by the thickness of the MDF rather than by something you need to measure or cut, so it is consistent. For pinhole that won't matter though. With mine, the trick Jim mentioned with paper is also needed to remove the paper negatives, but it works like a charm ( thanks to my schoolteacher wife who suggested it to me... they know all the little tricks like that! )
There is a picture of some of the pieces before I glued them together in this post:
(there was a url link here which no longer exists)
I used the "soft side" of inexpensive stick-on velcro strip as the light trap and it works just fine.