I've thought about doing this myself but was wondering about how to deal with filters: since the pinhole is so small and likely close to the filter itself, will flaws in the filter be magnified?

I have two pinhole "lenses" for my Mamiya TLR but no filter thread and I have manually held a filter over the pinhole when necessary. I'd like to try using a polyester theater gel instead (taping it to the inside of the pinhole), which brings to mind the quality issue...