5260 leaky gas cap

First of all part of the problem is that the cap popped off the tractor while I was mowing and... you guessed it! It went through my 40 twin blade mowing deck! Ouch. See the pics. It was pretty mangled but I managed to get it back on the tank. However now gas sloshes out around the rim of the cap. I can only imagine it's now deformed and is not mating well. I tried custom cutting a rubber washer but that just got all bound up in there and didn't seal well. What else should I try? Can I get a replacement cap somewhere? Please advise.