You can have it machined, or go with a redneck's barnyard engineering.
I took a piece of 1x4 pine about 4" long and glued a piece of cardboard about 0.020" thick from a cereal box with a hole a bit larger than the o.d. of the seal. Put it over the seal and tap the seal in until the cardboard touches the case.
2 years and 10,000 miles and no leaks.