A resourceful Redditor posted this nifty round count indicator modification they did to an AK-47 magazine. It’s so simple it’s actually pretty brilliant.
Open up your mag and drill your hole first. Put it back together and load 6 rounds. Make scratches to indicate which part of the follower shows, repeat this with 3 rounds. This will tell you where the diagonal needs to be. Put a base layer of white, the use masking tape to mark your line, paint the yellow, switch your tape to cover the other side and paint red. Clear coat to minimize wear and there ya go.