Which would be the hardest one? Maybe '+'?
I don't know. I just tried + and got it similarly with 3 whites around it and 5 blacks around the whites. Similarly I got a "bulky 5" easily with 3 whites and 5 outer blacks.
(They surely all need 3 whites around the center black, and if it can be done with only 4 outer blacks, I'll be impressed!)