A friend of mine built one that is shown in DOTO's link. It looks awesome
For my creep, I picked up a big planter at Walmart a couple of years ago with similar shape to yours. It's a dark burgandy, so I didn't even bother painting it since it looks great at night. While it doesn't have the traditional cauldron shape, I don't think anyone notices because they are focused on the creep.
I saw a hose holder at Sams the other day that looks like it would make a great cauldron but it's a little pricey.
Looks good. Just a suggestion, if you build your "fire" using the traditional spray foam method, leave the cauldron flat on the floor and build up the foam around it. This will give it more of a "cauldron" appearance by reducing the height of the planter.