The keg seals on a Cornelius keg seal under pressure...
They are cheap and have other uses.
You cut a small hole in the Sankey keg (or just use the corny) the
size of the pressure O-ring corny seal. Slip in the seal and clamp
it down. You will need a small hole for the airlock. It's clean and
cheap, and you will always have a corny keg for beer making.
If you have a keg that has a large hole in it already, then you have
a way to seal that hole with a bowl, most likely.
My 'bowl' has been fitted to a 2" copper female thread that I can
screw a plug into with an airlock.
The idea of a Sankey keg as a primary fermenter is explored in a
beer making dvd torrent. I forget what it is called right now, but I
can find the name if you wish. I can also email you pics of the bowl