You can check around engine shops or truck dealerships, they make a rubber stopper/plug with the washers and nut preattached to the rubber plug, and as mentioned above, just find a machine screw with a tapered head to fit the nut in the plug, and tighten it up. As it tightens it compresses the rubber plug causing it to expand and make a tight seal in the hole. They are used all the time as a tempuraury field fix in engine blocks when a frost plug blows out or you loose your drain plug. Cummins, Cat, Detriot, etc used them on dipsticks to seal off the dipstick in the tube to prevent dirt and crap from getting into the engine.
I like using these rather then building one with Home Depot parts cause it is one unit, but not sure on price difference.
