The seal is quite specific, not so sure if a DIY substitute would be easy to make (though I never tried, just my gut feeling). The seal sits in a groove on the driver, I think I would try to cut something to shape from a bit harder plastic (i.e. Coke bottle, something that would stay seated in the groove). You could also try to get a hold of G and ask him politely if he could hook you up with a new one, he might be in the Christmas spirit
Alternatively the driver will function fine without the seal, you'll just collect dust and rinse out the bearing grease a bit quicker.