Ambivalent posted how it was done for that afghan in the photo section of the CAL. I'm just quoting
here, and thanks Ambivalent for providing another edging option
"ACTUALLY... I've changed my mind. I sat down with a piece of paper and a pencil and drew out how to make this work. I guess I'll be tearing out my progress on the border to date and starting again.
R1: sc around as in the official clue - with 2 small changes. Instead of 13 sts per side, you need 14 (so slip an extra one in somewhere), and instead of 2 sc in each outside corner, you need 3.
R2: sl st over to the middle st of the corner. Ch3, 2dc in same sp as ch3; *ch 2, sk 2 sc sp, 1dc in next sc* repeat until you get to a corner. For inside corners 3dctog, for outside corners do 3 dc in the same sc sp.
This should work *fingers crossed*"