DMG p. 255 provides the stats for siege equipment, and lists suspended cauldrons among them:
A cauldron is an iron pot suspended so that it can be tipped easily, spilling its contents. Once emptied, a cauldron must be refilled — and its contents must usually be reheated — before it can be used again. It takes three actions to fill a cauldron and one action to tip it.
Cauldrons can be filled with other liquids, such as acid or green slime, with different effects.
Boiling Oil. The cauldron pours boiling oil onto a 10-foot-square area directly below it. Any creature in the area must make a DC 15 Dexterity saving throw, taking 10 (3d6) fire damage on a failed save, or half as much damage on a successful one.
I'm trying to figure out how many flasks of acid (a flask contains 1 pint) I'd need to fill this siege engine. It's mostly a measure of cost, and if that would change the damage. If so, how it might.