Here's one done with a lens that sounds identical to yours.

It was the front light on a Korean War era spotting scope that a spotter for gun mounts might have used. It was in the trash, and I retrieved it and machined a 3" ABS male adapter to recieve it. The pipe threads on the male adapter just start into one of my Cooke Portrait lens flanges so it's easy to mount on the Kodak 2D 8X10 with Packard inside.

Good soft focus doesn't need to be expensive.