Actually, 7x10 sounds like a nice negative size!

But if you could find a old wood 8x10 negative (or plate) holder for a low price, then you could construct a simple camera around the holder. I have seem 4x5 backs held on with rubber bands -- 8x10 might need pretty hefty rubber bands! An old plate holder would be nice -- use a low-tack double-sided tape to hold the paper in. The plate holder, unlike film holders, does not have the side channels that crops a little on the sides on the negative. But if you come across a cheap film holder, go for it!

Take a look at these --especially the 8x10, the view of the back. One of the possibilities.

http://www.pinholecamera.com/prices.html