Never mind either the quality or the thickness

[Bob Kneale]

It is also possible (as an emergency quick fix) to use a 'brassed' drawing pin instead of the washer. It is especially useful if the wire lead has been soldered onto the washer by a well meaning owner and has fractured. The pin can be pushed into the lead and you're on your way again. I've occasionally forgotten to renew the connection and driven around for many months with it although I wouldn't make a habit. It's a much cheaper solution to buying the 'proper spring washers', and to could always drill a small hole in the head of the drawing pin.
Just a thought.  Bob