Sandy -
Not aware of any, but it's not rocket science to DIY. I've built a couple of sloshers for 4x5 film - a six sheet model for use in 11x14 trays, and a 2 sheet model that fits into my 8x10 trays. It would be relatively simple to configure a 2-sheet model for, for example, 5x12 film that fits into an 11x14 tray.
I think the greatest concern would be to make sure that the barriers are open enough to allow fluid to flow freely between compartments. The barriers in 4x5 models tend to be solid, with gaps at the corners. If I were building a 5x12, I think I would include several notches along the bottom of the barriers.