Lot's of us use sankeys as fermentors. I'm pretty sure you'll want a #11 size drilled stopper. Some people prefer to stretch an orange carboy cap over the top, which works well too. I prefer to use a sankey coupler with the check valves removed and the beer out blocked. I either connect a spunding valve to the gas side, or run a hose from the gas side into a blow-off jug.