Replacing toilet plastic motor shaft

I'm afraid that I misplaced the small plastic coupler that connects the Thetford motor to the pump assembly. When I called Thetford, the guy told me that they didn't sell the plastic piece separately, and a new motor assembly was $260!!! I have to think there's a way to rig up a new shaft to replace the 25-cent part from the factory. It looks like a little piece of PEX tubing... has anyone had to do this before?