When you kill Professor Slate, you can loot an Empty Polyformic Acid Vial. Now, you can use the Empty Vial you looted to "store" your disguise creating a Nearly Full Vial of Polyformic Acid. If you later use this vial, you re-gain the disguise and get replacement Empty Vial. This process effectively gives you 3 uses because each time you store and release the disguise, it becomes less effective (as it becomes Half Full Vial of Polyformic Acid and then Nearly Empty Vial of Polyformic Acid) and you're eventually left with an Empty Vial and no disguise. You'll get a message something along the lines of "There isn't enough acid to store...."
However since this effect persists through death, when you res the buff will return to a full 60 minutes and you will be able to store it back into the bottle and get a Nearly Full Vial of Polyformic Acid. Tested only through dungeons but I believe it should work anywhere.