Seems to fit the drain plug on most sea kayaks I have played with.

Sweet features:
-> Loop a lanyard through the hole on top to keep it from floating off into the nether.
-> The gear pattern around the base should help you torque it down nicely.
-> There is space at the base of the threads to throw a gasket in. (exacto blade + rubber sheet type adventure here....)

Printing in PETG with 100% infill & the cap on bottom and threads on top seems pretty solid. let us know if anything fails out there if you give this design a crack.

Enjoy!