I ended up getting some cheap 4X5 holders, so I'm changing this build to the smaller format.

With the film holders in hand, I'm basically going to follow this tutorial. http://www.stanford.edu/~cpatton/foamcore.html

Couple questions:
-what is a good focal length to start with on 4X5?
-on this build, what is the purpose of the foam core back? Also pictures of it at the end of this link http://www.stanford.edu/~cpatton/dai.htm. If I'm using a film holder, do I need it?

Once I get these questions answered, I'll hopefully draw this out in my CAD program and cut it at work at the end of the week. Then all I have to do is assemble the pieces and make the pinhole.