One option is to have several UOMs for various drum volumes.
For example:
DR015 - 15 Gallon Drum - 1 DR015 = 60 QT
DR030 - 30 Gallon Drum - 1 DR030 = 120 QT
DR055 - 55 Gallon Drum - 1 DR055 = 220 QT
You could then order Qty 1 DR055 of some liquid and track inventory in gallons or quarts.
Depends on how many different drum sizes you are dealing with. If there are too many options, this doesn’t scale well.