I use orange braided line- Suffix 20lb test. Don't laugh- it's the same diameter as 6 lb mono and doesn't kink in the winter- I use a black Sharpie to mark it at the depth I want. I black it about 3" and you can see it on the reel as your line pays out. I usually mark it at 28 ft- that lets me keep the black mark just out of the reel and fish 20 ft. deep with an 8 ft. rod.
Most of the small ultra light open face reels also have a little hook on the back of the spool where you can loop the line and it stops line from paying out. It's not foolproof and it's somewhat difficult to hook the line when you're wearing gloves. And if you want to go a little deeper, you have to get it unhooked and let out line and rehook it. But it does work. I've tried the bobber stops, but the simplest is the Sharpie mark on the line. Using 20 lb. line, I don't have to retie very often.