I built a pinhole with 4x5 film holders and it works great. I have a 4x5 enlarger so that's what I went with. A resource I used was on YouTube and it was a 10 part series on building a 4x5 pinhole camera. Just type that in and it should show up. Of course everyone modifies the instructions to fit their own needs and intuitions and I made a few myself. Anyways, this will give you an idea on a design for a holder style pinhole and you can just size up the basic concept to fit an 8x10 holder. A lot of the stuff he goes through is just basic woodworking so you can fast forward through some of the parts. Have fun.