You have a couple options. You can get a new, shorter nipple. How far are you from getting it to tighten? If you're 1/2" away, get a 1/2" shorter nipple. If you're close to getting tight enough by like 1/4", you can get yourself a bunch of washers to built up on the outside of the cooler. They can be plain old steel.
edit: Dang, too late.
Sightglass, Refractometer, Ball Valve, Weldless bulkhead, Thermometer, Decals, Stainless Steel Fittings, Compression Fittings, Camlock Quick Disconnects, Scale, RIMS tube, Plate Chiller, Chugger Pump, Super Clear Silicone Tubing, and more!New Stuff?