So, after scouring the interwebs looking for some sort of kludge to allow me to attach filters to my Diana, it occured to me that I could add them behind the lens instead of in front. I cut circles out of a Rosco Cinegel sample pack and they fit perfectly between the camera body and the lens. It's stupendously cheap. Super simple (and compact) and it still allows me to adjust the focus of the lens without having something in the way.
Hurray for cheap solutions!