After my next fill-up, I had the strong smell of gasoline when I pulled back in the driveway. Upon inspection, the cap was clearly not sealing and plenty of gas had sloshed around and out the top. It was puddled behind the fuel door and had run down the side of the car.
I put the old cap back on for the time being. The flimsy little rubber gasket that came with the new cap was way too thin to make a decent seal. I started googling around and found that lots of places sell new gaskets for ATVs and lawn tractors, but nothing I could find specifically for this size of cap. The best thing I found was AutoZone sells rolls of “rubberized cork” gasket material (AutoZone link). Oddly, the package even comes with a utility knife. Also, to be fair, O’Reilly also sells it, but is a couple of dollars more.
I traced out the thin gasket that came with the new cap and cut out a couple of new gaskets.
Two layers of cork gasket ended up being too thick, but one layer of cork and the thin rubber gasket was enough for a nice, tight fit. I haven’t had any additional issues with it.