it wouldn't be hard to make one, try a hardware store for a o ring making kit. cut the right size piece of rubber to length, cut the ends on angles that will meet for a better seal. glue ends with superglue. we do this all the time at work when we can't get the right size seal quick enough and it works fine, the only problem is holding the ends of the rubber in the right position while the glue dries without sticking your fingers to it
also, cheers for the wiring diagrams coltturbouk, they're coming in very handy.