I've done a 4x5, and this year, an 8x10, that were done as rather serious woodworking projects.
4x5 box
8x10 monster

The general idea of the 4x5, a rectangular box with three sides extended in the back to hold the filmholder could probably be imitated/adapted in foamcore (and scaled up in size to 8x10). I doubt you could use rubber bands the way I did to tie down the holder as they generate a fair amount of stress. Maybe some sort of elastic (Velcro?) that wraps around the whole box would work.

It might be worth considering some 3/8 or 1/2 inch square "sticks" - maybe available in basswood at hobby shops -- glued into some of the corners to toughen corner joints in a box big enough for 8x10.

On the 8x10 I used six inch pinhole-to-film spacing and a 22 mil (.56 mm) pinhole. Gives about a 92 angle of view at the diagonal -- pretty wide but not outrageous fall-off.

Another APUGer's effort at 8x10 is pictured here but I'm not sure of all the details.