We picked up a larger cauldron for the witch's area this year and I decided to modify it by combining ideas from Dave Lowe, Through My Looking Glass and Make It Do. I ran into some problems with this one so it's not exactly how I envisioned it but I'm still pretty happy with it and I can improve upon it next year.
I started by patching the seams with spackle and then sanding it down. You can still see the seam but it looks more like an weld now rather than poorly made plastic. I drilled holes in a couple of styrofoam balls and glued them in the spot where the original "handles" were. The rings were made out of model magic that I rolled into a snake and then formed into a circle.
After the rings dried overnight, I ran them through the balls and sprayed everything down with textured black paint. The styrofoam that I picked up turned out to be the kind that melts when you use spray paint on it so they look a bit wonky but I kind of like it. Once all of that was dry I sprayed insulating foam under the rim of the cauldron. That worked alright but trying to cut it down and round it out was a bit difficult. Then we decided that we didn't really want to deal with keeping the dry ice going and I am allergic to the fog machine so it has to stay outside. After thinking about those issues (and my lack of skills in sculpting the rim), I decided to just go for the bubbling effect instead.
I filled the cauldron up with paper until it reached the rim. Then covered it all with Great Stuff insulating spray foam. It took about an hour to dry and expand (it doubles in size as it dries) then it was ready to go! The cauldron is set on top of a twig wreath that I picked up at Goodwill, along with some orange tissue paper to look like flames. I will probably try lighting it from underneath to make the "flames" glow but I haven't found lights that are bright enough yet.
Hopefully you won't be able to notice that run in the paint once the lighting is dimmed. Next year I will probably find larger balls to better fill out the original handles...and I will use acrylics not spray paint. :)
This is actually a really simple project so if you learn from my mistakes you should be able to make yours pretty quickly and with even better results!