You can buy a small, cheap cutting board (HDPE) from Walmart. Dollar stores sell them but they're small and way too thin. A dremel tool with a milling bit as long as you're careful, you can cut a circle out of it and then make a circular channel, maybe half the depth of the material thickness to sit over the rim of the jar you're using and just drill a couple holes to push the electrodes through. The only thing you'll have to deal with is how to secure the electrodes. Friction works but plastic wears over time and expands and contracts with changes in temperature where its better if you figure out some way to secure them.