Brew man, why not cut the top off a corny keg and use that?
The other thing I'd look at is turning the thing upside down and fit to the bottom.
Because the corny costs $20 and a welding shop wants about 1 hour ($75) to pressure weld a Corny top to a Sanke. I don't have tig welder.
I think I can accomplish the same thing by cutting a hole and putting a plate over it. And if I can't, I can always get a corny welded in place.
And, the corny hole is ok for a corny, but it would be nice to have something just a bit larger, at least for my forearms.