(OK terrible photo, but let's just ignore that...)
LOOK MA!! After only 8 attempts... Seriously, I started this block EIGHT TIMES before I got it going. It kept curling up like a cup on me after two rounds. I was about to lose my mind. Not only that but my clusters (which you can see in the centre and extending out in an X) just didn't look clustery enough. So I increased them from a 3tr cluster to a 4tr cluster, and the corners in the centre round and the 1st round up from a 2-ch to a 3-ch, to give the block a bit more space to turn into a square, and not a round.
So, 4 out of 5 done now : )