Designed to fit M12 threads. I used it on a Subaru Outback with an M12 x 1.25 thread.
The hole is sized to properly fit a Subaru and the collar at the bottom will hold up a Subaru shift boot not sure how it would fit on other designs.
The threads are extremely tight at first. I did this on purpose as I didn't want a loose fit. If you use some tri-flow or other lubricant to "tap" the threads it works pretty well.
This is designed to be printed with support only around the boot retainer at the bottom. I found that setting Cura to a support angle of around 85 degrees would support the needed area around the base but wouldn't print anything inside.
Post printing I sanded with 200 grit sandpaper then used 3 coats of high build primer wet sanding in between with 400 grit. Then a few coats of matte black and matte clear coat with some 1200 grit wet sanding in between for any runs.