Weld or solder a 4" Triclamp port on the bottom of a Sanke. Turn it upside down with the 4" port on the top and the 2 inch on the bottom. Equip with chiller coils, air locks, racking tubes, etc. as necessary.
The good thing about Sankes is that they are totally airtight and they handle pressure. ANd they are cheap. They aren't a conical, but it is very easy to rack from one to another if you want to use one as a bright tank. Beer needs to stay on yeast for a while anyway. If you use CO2 to push the beer, there is no oxidation.
I can fit my arm though a 4" triclamp ferrule to clean the inside of the keg. It is about the same size as the opening on a corny keg. I ferment with fruit, so getting access is necessary.