Air tight, definitely.
You also ways the piece of plate that you use to be thick enough to endure some pressure. Not crazy amounts of pressure, remember that the exhaust is open at the back. Maybe 10 psi or so. If the plate of aluminium is too thin, maybe double it up. The more elaborate it gets though, the higher the chance that there will be a leak. All you are doing is blocking a hole. How hard can it be? There is even a bolt hole on either side to assist you.
Worst case scenario, use the standard pipe that is on the head and block the tube off by welding a bolt into the hole, perhaps? It doesn't need to be pretty, just needs to be airtight.