Vernier Caliper Style measuring device?

So I'm looking for a Vernier Caliper style measuring device in inches. Really what would be perfect would be something say the size of a T-Square you use in drywall (maybe a bit smaller) that functions like a caliper for measuring the outside diameter of objects that could be up to say 26" in diameter. Actually, an add on piece to a drywall T-square would work as well. Could be made of plastic. Measurements to the 1000 of an inch is not the accuracy I'm looking for but something that is going to give a pretty close measurement. I can't find anything online that resembles what I'm thinking of.