From a few dimensional snaps with camera and measuring tape, I created a model in Fusion 360. So far I have eyeballed it from the photos, but if precision is required, exact pattern could be made from MS Paint. I can always sand the piece until it fits exactly. Test fitting tonight, once I manage to get it off the build plate. Result: perfect fit. Also decided to model a variant with a short pipe flange.
Thanks, it's quite strong at 8mm thick, confident it would support 500 lbs, and the 0.2mm layers give it a nice texture for a foot massage!