When I put together rituals I had a Baphomet flag/tapestry I hung behind the altar. I've been experimenting with shaped canvas and would like to make a nice big sigil but stretching a perfectly round canvas is extremely difficult.

If you're working with a round piece of wood, merely painting it might look a bit cheesy, like a prop in a high school play, so I would suggest etching or better yet burning the sigil into the wood once it is traced. Word burning tools are fairly cheap at Michaels craft store and they're always running specials!