The best way I've found to amass ashen keys is to simply go fishing for pondies!
During the day, an inland pond's loot table looks like this:
4 varieties of Pondies (charcoal, orchid, bronze, bright),
4 varieties of Forsaken loot (old hat, old boot, old skull, fish bones),
Planks,
Ashen Keys.
At night, you also have to contend with Moonsky Pondies, so for best results fish for keys during daylight hours
EDIT I actually just realized that fishing for splashtails (casting witout using bait) at any small island would produce similar results, since islehoppers don't spawn at small islands.
EDIT Fishing without bait in deep water is a third way to minimize the loot table and maximize keys