The plug on my Coleman chest cooler broke, and the few that I found on Thingiverse didn't fit my cooler, so I made this one. It snaps over the cooler drain nipple quite firmly, and you can add a rubber o-ring for an even better seal. I designed it in OpenSCAD with initialization variables that you can easily tweak if your cooler's nipple has different dimensions, or if you just want to tweak the fit.

Monoprice Select Mini

Doesn't Matter

No

.2mm

20%

.8mm walls

The plug is a snug fit without the o-ring. A leak test generated a slow drip when the cooler was half-full of water, so I added this o-ring and the leak stopped. I got this at my local mega-hardware store for about $2.50 / 10-pack.