Any economical solutions for filters used with lenses like Symmar 5.6/300mm or larger.

I have pretty much adapted Cokin's P adapter with gelatin holders during last decades but it offers adapters only up to 82mm diameter - which is too small, even for my largest MF lenses.

For now, I have only two ideas. One is to built own "adapter" like this and use cokin's filters in hope that they won't get scratched.
Of course, there's no way to use polarizing or graduated filters with such DIY adapter.

Another option would go for another system which gives basic setup, adapter + filter holder cheaply (is there such system, Cokin's own Z and X systems aren't cheap, in fact even Lee is cheaper).
Going for hunt bargain filters from Ebay for eg. 95mm diameter and use these with step up rings?

EDIT: How about using filter attached to back of the lens? Assuming that there's enough space in the bellows?