3/4” piece of copper tubing, sharpen it with a rat tail file. Then you can cut them out of the styrofoam foam trays that your meat comes on from a grocery store. Cheap and easy!
Using foam of any kind makes me leery of heat (shouldnt be an issue for over shot use unless you are really warming up the barrel with many rounds) and petroluem based chemicals causing a nasty reaction
Test your substitute foam with your cleaning and preserving stuff.