I built my 10-gallon rubbermaid home-depot cooler today. having all the part numbers was a real breeze, but i must admit I was a little confused about just where the o-ring is supposed to fit. the opening in the front of the cooler is way bigger. I ended up pushing it back in between the threaded nipple and the stock spigot seal. I also got smaller 5/8 washer for the front that would fit in the indentation on the front, so they would help keep the whole arrangement centered. for the stainless washer, I got a metric one that would just barely fit on the threads of the nipple, then dremeled it out a tiny bit. It took a couple tries of putting it together and guessing how tight to make it, but when i made it 'fairly tight without large tools' it stopped leaking, so i'm tentatively optimistic.
had to make 3 extra trips to the store, one because the stock seal on mine was caught in the threads and damaged (poorly assembled by mfr), and another to exchange a washer that wasn't the size it was labeled, and one final trip to get a couple smaller fender-washers for the front. If it works perfectly from here on out, I'll be semi glad I spent a few hours to save 50 bucks or so. If not, then at least I learned something.