get some (empty) small travel size shampoo bottles and stuff some open cell foam into it after filling with a small amount of thinned enamel paint.
Trick is to get the foam stuffed in there with a small amount hanging out (that should be trimmed to a square or circle as needed, depending on mark) and have the right paint consistency.
Practice on scraps until you can quickly dab the foam and ger the right paint dribble. Try having about 1/4" of foam protruding above the bottle top, that seems to be the right amount. Thinning is tricky, it does not take much.