I am going to go buy one. I have one and have no clue where it is. I will find it after I buy another one.
It’s what you use to measure diameters. Sounds like this likely use this is a one off in your case. It’s best if you can borrow one from some one you know with a well stocked tool chest, like a machinist or auto mechanic.
If you need to buy one for measuring strings once or twice, try a store like harbor freight, or Northern Tools, or Amazon. You don’t need to buy the best one on the market as they can get pricey. Oh, and buy a small one. Best wishes.
BTW, It’s pretty cool having a son pick up the bass as an instrument of choice. My son started playing at age 13 and has made a career as a bassist in the Marine Corps, complete with a never ending stream of gigs, steady income (which is a lot more than what one would think) , free health insurance, housing, tuition reimbursement, GI bill, and a retirement plan.
Me? I starved for my art…LOL. Seriously, it’s a great gig if you want to be a true professional musician.