Using the engine mount position means that you cannot fit the fixture to a complete bike. The one I made as part of a college project about thirty years ago braced from the front top sidecar fitting to plain pegs in the unused threaded lugs on the RFM rear forging. I remember using thin wall rectangular box section, approximately 2”x1” with round tube spacers either side on the stud through the sidecar fitting. A single tube spacer held these box sections apart under the spring boxes and I seem to recall some uprights, probably from the long stud to carry the rear top sidecar lug. I built it for a Comet but to be universal I believe that multiple front holes and at the bottom of the upright struts May be needed. I think the rear pegs were 1/2” bar threaded one end for nuts either side of the box section.