I'd be surprised if the Horseman will not fit with the springs removed and using bungees. I plan on using the DaYi back which is multiformat 6X6 ti 6X12 and fits Graflex back. The measurement for the Horeseman should also fit a 4X5 Graflex. The Horseman is simply an overpriced roll film back, but measures to the Graflex opening that uses DDS, so should fit this back opening (sized to DDS as it is) and held on with the cords.

Can't imagine it's not a surmountable problem. The DaY is half the price of a used Horseman, in any event and multi format to boot.