How about a half batch version? You could base it on your 2.5 gallon Cornelius keg, replacing the bottom dome with a cone and ferrule. With the kegs, butterfly valves and collection jars already being produced you’re already 80% there! With one litre in the jar and another in the cone, that should leave enough headspace for a half batch and end up being about the same height as a normal 5 gallon keg.
Plus, a uni tank based on a corny keg would have to be called a uni-corn. It practically sells itself!
Full disclosure - this is actually my standard batch size and I’m motivated entirely by self interest.