I would NOT use nylon for a washer. Nylon can soften when warmed and then the nut will be loose and the connection isn't secure. Resistance goes up and then there will be a problem at the connection.

Use a material that does not flow. A fiber washer is good, and there a plastics that are acceptable -best if thermosetting, but in any event not something to risk with a maybe OK. Something from an electical supply house should have the info needed.