Greg - AH - I will be the first to admit the "blob" does not sit flat on the disk harrow. It backs up against the cross beam in the back and rides up that support member you mentioned. I've used it in this fashion several times and never had the "blob" fall off. I suppose I could have made some sort of allowance for the middle support when I poured the concrete. However - it probably would have compromised the strength of the "blob" itself.
Hey - this is not rocket science and even though pulling that disk may make me look like the Beverly Hillbillies - there is nobody within miles watching. And the overriding factor - - it works real well. I would not want to go beyond this much weight either - the disk is only a light weight class 1 implement.