This is a frame-mounted luggage plate for Brompton folding bicycles. I printed mine out of a carbon fiber infill polycarbonate alloy, but other plastics should do fine, as I designed it to be reasonably robust. To mount, it uses two ¼-20/M6 U-bolts with a 1¾"/45mm internal diameter. I slid heat shrink tubing over mine and shrunk it down to achieve a surface that won't mar the bike's finish. I had to cut the U-bolts down to size, as finding exactly the right length was not possible. I would suggest getting either a thread cutting die or hardened nut of the diameter you need. Thread the die or nut on before you cut the bolt. I'd use a rotary tool (dremel) or angle grinder to make the cut, then round off the cut end. At that point, you can thread the nut or die back off to clean up the threads.

The U-bolts won't be cut to the same length; the back one will be longer. Measuring from the inside bottom of the U, I got 49 and 63mm, respectively, though there's some wiggle room.

Ensure that you print with a 3.5-4mm wall thickness for maximum strength.