Tapered screw for plugging brass plugs / fill test?

I've got a couple of brass cleanout plugs that have the small threaded holes in the center for attaching a wall cover.

Is it good enough to dope up the screws that come with the wall covers to seal the hole for the fill test, or is there a tapered screw I should be looking for that is more watertight?

Put a rubber washer under the bolt head and tighten it in.

I LOVE no-nonsense, common sense solutions.

Thanks, hj.