Most useless photographic accessory

[MTGseattle]

I don't know what the Linhof filter holder looks like, but a thing you can use for lenses without a filter thread like those older Ilex-Calumets, is a spring tension clamp gel filter holder with a 2-barndoor hood. The one I know is called a "Voss gel filter holder" like this: https://www.ebay.com/itm/235439128743

It holds 3x3" gels, but you may be able to slide a Cokin filter or similar into it. I have one of these. It doesn't fit this thread because it is cheap and occasionally useful - most usable solution for odd-size clamp-on hood that I know of. Pic from the ebay auction for posterity:

View attachment 422729

Interesting. I have heard of 2 square holders of this basic type now. I think the other one (Cokin maybe) uses rubber bands.