is a Fender washer mandatory, or would a flat washer work?
All I need is that O ring (which HD carries but were sold out of) and I'm good to go.
I just finished mine, and had first tried to get these parts, unsuccessfully, at Home Depot.
The problem is that you need a washer with a 5/8" hole, and a maximum outside diameter of not much more than 1", or it won't fit on the inside of the cooler (the spout hole is close to the bottom). And it has to be pretty rigid and thick just to do its job of holding the washer down tight. I don't know if "fender washer" specifically means something with that small outside to inside diameter ratio and still fairly thick... but if it does, then you need a fender washer.
Other people with access to shop tools have started with a flat washer and have drilled the central hole larger.
Others have mentioned bargainfittings.com, which has the washers and your o-rings. And these parts are a lot easier to find on bargainfittings.com than on McMaster Carr's website. That's what I'd do if your local option doesn't pan out.