Build a motorcycle seat

Step 9 Mount

If you're going for a quick-detach seat like I was, you'll be using five nails as locating pins. Place the seat in the proper position on the bike (sit on it to make sure... maybe even take a tentative lap) and reach underneath with a nail. Mark the locations for your pins one at a time by pressing the nail into the tape, in such a way that the seat can be installed and removed easily, but with minimal other movement once it's installed.

Some movement is unavoidable, but not an issue once you have your weight on the seat.