Working with Adobe Illustrator, I traced the side profile of the bike and made a simple line art model that allowed dozens of tweaks before confirming the desired proportions and position of the bum stop (also roughed in the plan for the rear swingarm bracing and shock mount).
A friend of mine is an artisan when it comes to shaping aluminum. He will be sculpting the tailpiece, fabbing the subframe area, and welding/bracing up the rear swingarm and shock mount. He’s a very meticulous fabricator. I wanted to try developing my own shape for the tailpiece and he advised using 2” blue foam insulation board as the "buck" or pattern. So, I picked up a 4x8 sheet at Lowe’s and gave it a shot, check out the photos below. Tools used (as recommended): scroll saw, big toothy kitchen knife, 3” drywall screws, wood rasp and foam board glue. Fun!