As is my habit, I am making a complicated engineering and woodworking project out of an 8x10 (in between a bunch of other things trying to happen). There are a few more design issues to be addressed, but an oak frame that establishes the dimensions and secures the filmholder is sitting in the workshop with glue drying. Nothing like last minute! But then I do have a functional 4x5 if the new project misses the deadline.
(Hmm -- compound miters -- yeah, I must be crazy. )