This thing bolts onto the rear of a bike to carry a fishing pole. The hole spacing is 27mm and the main ring is 30mm in diameter.
Since it is a two-piece design, you can make other attachments too.
More details listed on the instruction tab.
Use standard 5/16" hardware to mount to the bicycle. Most bikes have threaded holes near the rear axle.
A 3mm x 20mm screw holds the parts together. I recommend using a tap to make the proper threads. There is a lot of torque and stress in that area of the assembly.
Prints together in 30 minutes on my machine at 0.4mm layer height and 40% infill.