I'm also doing a teardrop project(i'm lucky i have nice lugs and complete) that came to me with regular 12-24 inserts on the toms with square headed tuning screws. I am converting them over to the traditional slotted 1/4-20 sonor screws and thought i could just go to the hardware store and pick up a few 1/4-20 nuts. Boy was i wrong. I went to every hardware store in town, and looked all over the internet and no standard 1/4-20 nut fits inside the lug. I measured the hex at 10 mm.
Standard hex size is 7/16", i just happened to find some in a bulk package that should work just fine. These measure at 3/8" which is about .005" smaller than 10mm. BTW i'm in the US so metric countries may not have this problem.
Good luck with the project!
Gary.