I kinda feel like I am spinning my wheels with my project. Seem to be jumping all over and not accomplishing anything solid. I have an after market frame with a 8 inch Z in the rear and a supposedly 4 inch stretch in the front. My 29 coupe is totally apart, every panel, pillar etc. I have a sub rail assembly that I removed the rear cross bar so that I could slide it down to clear the Z. I need to modify the sub rail to go over my rear axle so the whole assembly can lay flat on my frame. I'm not totally sure about what the best way to accomplish this is. I was messing with a piece of junk steel I had laying around trying to mock something up as a template. Is there a better way I'm not thinking about here? I want to get this Sub rail down on my frame so I can start piecing the body back together again.
Here is one idea; but you would have to shape the tube to follow the bigger Z in your frame. There is also flat piece of steel added to outside of the tube that extends to the lower lip on the wheel well and is later trimmed to match the inverted U axle clearance cut-out and welded. The fuzzy second pic isn't a coupe on an A frame; but the lower edge of the wheel well panel and axle cut-out would be similar.
Patience, patience, patience. ... As for mockups, call a local real estate company and ask them if they've got some leftover corrugated signs or in the worst-case check out a hobby shop on even Home Depot and pay retail for their corrugated poster board made of plastic. You could even just use cardboard. Plus a roll of duct tape or Gorilla Tape. For smaller pieces or for Sheet Metal mock-up get yourself a box of legal size Manila file folders. For carpet or sound deadener or padding get a roll of painters masking paper from Home Depot. tape the pieces together press down in the area. measure and Mark with chalk. Cut out at the chalk lines and lay it on your carpet or padding or sound deadener pieces for perfect fit. Here is a link scroll down about three-quarters of the first page to see the cardboard templates to fabricate around my Jaguar rear end mountings. https://49fordcoupe.smugmug.com/Building-the-Frame/
More on using plastic signs for floor fabrication on weird shapes; Scroll way down. https://49fordcoupe.smugmug.com/Rebuilding-body/