I was rather hoping someone knew the distances between the marks on a bit of wood and would tell me so I can make one
Ooo, I wouldn't touch that with a ten foot dipstick! Pilot runs out of fuel - investigators ask: "How'd you know how much fuel you thought you were carrying?". Pilot answers: "I made a home made dipstick with dimensions I read on an internet chat group, from a person I do not know, for a model of 172 I could not confirm."
Best to make your own. Don't do it with the fuel selector to "both" or "off", and assure that the plane is sitting repeatably level, as that makes quite a difference in the reading. Cut the notches in the handle of the spoon, rather than ink, which will wash off.