So… The name Doppelbock comes from Bockbier which is derived from the town Einbeck where it originated. The towns name likely comes from a dialect version of “ein Bach” (a brook).

The best feminine name would therefore be Doppel-<feminine noun meaning something like brook>. But alas there are very few feminine German words relating to bodies of water. Best I can come up with is Doppelquelle (Quelle means spring), but that sounds like sparkling water.