f295 member greyhoundman has posted instructions for how to build an ultra-cheap filter holder for large format lenses:
Ok, I know someone probably sells an adapter for holding filters on a large LF lens. But, not around here, and I like to see what I’m buying.
So again I decided to build what I need.
I used, 1 thin CD case, a piece of PVC pipe and a 6-32 nylon thumbscrew…
Read instructions at the f295 DIY Photography forum