Just use your cosmetic wedge and pluck out uneven holes the size you wanted the mottle to be. Berry makers were used to make "single" mottle spots. (and just placed them all over, one at a time) making their own pattern and sometimes just a few mottle circles where the artist wanted them to be.
If you need to make a "one" spot mottle tool for an area, you can just cut a cosmetic sponge (wedge) into a smaller width and pluck out just one hole.
I use several different patterns/sponges for all of my mottling now. If you feel you need to get into a small area to mottle, just cut your cosmetic wedge
to a smaller size and make your plucked holes smaller too. (that is what I do). Mottle doesn't have to cover every inch of your reborn as on a real baby
some areas just seem to "fade away".
Or, you might like the smallest tool on this set from MacPhersons to make into a single mottle hole:
https://www.macphersoncrafts.com/en/online-store/reborn-tools-supplies/tools/mushroom-sponge-set-detail
Hope this helps :)