Don’t worry about getting a perfect fit. If the o-ring that you get ends up being too big, you may be able to get a more secure fit by using rubber pads (rubber bands would work, too) inside the o-ring. The rubber pads I’m referring to usually come with anything that attaches to a bike (like mounts for speedometers, sensors, head/tail lights, etc.), so you might actually have some spares lying around.
Have you tried your local hardware store? My hardware store has o-rings in many, many sizes. I buy them for use with my Sigma bicycle computer. I always carry a couple extra just in case.
Have you tried your local hardware store? My hardware store has o-rings in many, many sizes. I buy them for use with my Sigma bicycle computer. I always carry a couple extra just in case.
This is what came to my mind as well. Well stocked local hardware stores are high on the list of a cyclist’s friends… all kinds of nifty items from o-rings to metric bolts can be had in small quantities for a very reasonable price. Don’t try Home Depot, you’re liable to have to buy 20 of whatever item you want because there are no self-service bulk bins like at a local hardware store.