What you need is a fine point Sharpie, some zip-ties, AR magazines and ammo.
Put a round in the mag, slide the zip tie in the back of the mag, make a mark on the zip tie along the top of the round. Add a round, make a mark, lather, rinse, repeat until you're at the listed capacity of the mag.
I saw this on Arfcom and Marv made a couple sets, one each of 5.56 and 6.8 for the both of us.
Works on green follower USGI and 'Nam era 20 rounders in 5.56. Doesn't work on a Gen 2 PMAG because the zip ties Marv chose are too wide.
Works on C-Products 25 rounders and Barrett 30 rounders in 6.8.