One can also use an open ended wrench (15mm or 17mm if I remember
correctly). If you look at the wrench, the open end is angled to
one side (at least on my Craftsman wrenches). The outside prong
of the wrench with its smooth curve with increasing radius,
then makes a fine lever if you insert it between the head and the
tabs on the jugs (and one of those two sizes will just fit
in that space). This worked when all kinds of tapping and prying
with screwdrivers and gently tapping chisels in that space
did not...

dave