I made a piece that can be added to a cauldron to make it look like it is bubbling. This piece was made to fit around a person sitting in a large cauldron, so they look like they are submerged in bubbles or a hot liquid.
I took a large round piece of cardboard and cut a portion out of the top and the bottom, that gives it an axe shape. The size you cut this will depend on the size of your cauldron and the size of the person whom sits in it. One portion that is cut out will go around their waist or chest and the other section will go around their legs.