BAD ASS SCARAB

I have been rebuilding Mercruisers drive for about 8 or 9 years off and on with my business and have been trying to find a plug for the oil galley hole for the upper and lower gear case that I can either screw in or drive in that is 100% air tight. An ex freind that was a OMC and Merc Tech used to use a pencl and drove in in the hole then pressure tested it but the pencl always leaked. I then tried a golf tee wtih electrical tape on the end and drove it in the hole it was better but still leaked during pressure test. I found a self tapping bolt last year that worked fairly well on the upper but wouldnt work on the lower gear case.
Anybody got a 100% sure fire way to plug these gear cases so you can pressure and leak test? Kinda sucks when you are trying to check for leaks on your rebuilding and all the pressure is going out your plug. Looking for somthing that will work on OMC and Mercs.