Corny Fermenter Airlock question.

I've read a few threads here discussing the merits of using a 5 gallon corny as a primary so I figured that I would give it a shot. I've explored a few options for airlocks/blow offs and think I'd like to remove the pressure relief valve and put an airlock in the lid. I see that others have done this with a drilled stopper and I was just wondering if anyone knew what size stopper I should pick up to fit snug?

Thanks in advance!

Why not a blow off using the "IN" post?

That was actually my first choice but I only have one working gas disconnect and its currently in use. I called the LHBS and they are out of disconnects so I thought the stopper/airlock would be just as good.