There have been a lot of posts the past 10 or so pages about brass and lead content. QUICK NOTE: At HD, Watts has the same parts in lead free options, generally the same part number, but with a "LF" in front of it. For instance, the 3/8" female hose barb is LFA-298 instead of A-298.
I bought an igloo cooler for my mash tun. The rubber washer that comes with it does not work for a sealer, so I had to buy something else. I actually bought a large faucet washer and cut the inside out a little larger. I did a leak test, and it worked fine.
BTW: Igloo coolers have a larger spigot hole of 1/2 inch. So, the 3/8" ball valve does not fit. You can still make it work by purchasing a 3/8" O-ring. The O-ring will fill the space around the nipple, which actually creates a pretty good seal. Use the rubber faucet washers for the interior and exterior seals. One of the benefits of the igloo cooler is that it does not have insulation where the spigot is. That means if you have any leaks, there's no chance for the wort to get into the inside of the cooler. It also means you will not need the 1-1/2 nipple. A close nipple will work just fine.
I also did an Igloo build and noticed this too - of course, I bought all the parts before I got the coolers in, so I decided to roll with the 3/8" parts anyway and see how far I got. I actually didn't use an O-Ring at all - just the seal that came with the cooler - and had immediate success. The nipple doesn't fit snug in the hole, but once you center the seal that came with the cooler and add enough washers on the exterior of the cooler, it doesn't move and has tested well with 10 gallons of ~120* water.
Other notes:
1. Create-a-bolt at HD for the SS 5/8" washers.
2. I did, however, use an O-Ring to "seal" the female barb on the inside of the MLT to the 5/8 SS washer.
3. Again, the LFA-xxx options for the brass fittings were great finds.
4. It took 5 washers on the outside of the cooler to get the ball valve to secure tightly and hold everything in place.
5. It is possible to tighten things too much, causing the cooler seal to fold/warp outwards from the SS washer. As long as nothing is loose and you aren't leaking, you're fine. If you are having trouble with a bulging seal, try loosening it - if it's still not moving it may actually improve things.
6. More of an FYI - I have a FB and will be doing fly sparging.