Just finished up a 48qt cooler MLT. It's not really any different from any other MLT bulkhead installation, but I thought I'd post my exact parts list for someone wanting to do a conversion with this particular cooler.
I started with a 48 quart Igloo Marine rectangular cooler.
Parts list - everything was sourced from my local plumbing supply store and hardware store. Total parts cost was about $20.00
The outside parts -
1/2" brass close nipple, about $2.00
1/2" brass ball valve, about $5.50
1/2" male NPT - hose barb fitting (for drain), about $2.50
7/8" stainless washer - about $1.20.
Rubber or silicone O-ring to fit over the close nipple, about $0.30
The inside parts - I chose to go with a stainless mesh manifold. You could do anything you wanted by substituting the inside fitting and manifold to suit your needs.
1/2" female NPT - hose barb fitting (for manifold), about $2.50
Stainless hose clamp, about $0.85
Stainless mesh fill connection hose, about $4.50
1) Remove the drain plug from the cooler by unscrewing the plastic nut on the inside. Retain the "washer" and rubber gasket, you'll be using them.
2) Wrap both ends of the close nipple with 3 turns of plumber's tape. Thread one end of the nipple into the ball valve.
3) Wrap the male NPT hose barb with 3 turns of plumber's tape. Thread into the other end of the ball valve.
4) Install the O-ring over the nipple flush with the ball valve. This doesn't act as a seal, it's there to take up slack.
5) Install the washer over the nipple so that the O-ring is between the ball valve and the washer.
6) Insert the pipe nipple through the outside of the cooler's drain hole.
7) Install the rubber washer you saved from the drain disassembly over the nipple from the inside of the cooler.
8) Install the plastic "washer" over the nipple.
9) Thread the female hose barb (or whatever fitting you chose) over the pipe nipple and tighten.
10) Leak test.
From here, I cut the stainless mesh hose ends off, removed the hose and used a stainless clamp to attach it to the hose barb. If you use some other kind of manifold system, substitute the female hose barb with whatever fitting you need.
You may need to use more than one washer to fill the gap between the ball valve and the cooler body.