+1 on the brass, my MLT fittings are brass.
I've got a brass nipple running through the drain plug, with rubber O-rings and brass nuts sandwiching the wall of the cooler, creating the seal. On the outside, I have a brass ball valve screwed on, with a 1/2" brass barb on the outside port to connect tubing to. On the inside, I have a 1" long brass nut, threaded all through, that screws onto the portion of the nipple inside the cooler, and allows the stainless mesh bazooka screen to screw onto the other end.