I am using the slot mortiser on my C3 31 for the first time on some exterior doors. I am using loose tenons. When I tried to clamp one of the rails perpendicularly to the mortiser, I discovered that the clamp arm is not long enough to reach the rail. Is my only option to build a one off wooden "bridge" that would reach out over the rail and be held down by the clamp? Seems a little too jury-rigged to me, but can't think of anything else.