If you are using bigger yarn -- like worsted weight, with a 5mm hook -- this version of shell stitch may make finger holes that are too big. In this case, use: (2dc, ch1, 2dc) for the shell stitch. To join on the last edge, use (2dc, sc1 in ch space of shell on opposite side, 2dc) to join.
Here is an alternate edging where it does not matter whether you have an even or odd number of rows in the square:
Ruffled edging: Ch5, sc in the middle of the block, **ch5, sc between this block and the next block, ch5, sc in the middle of the next block, repeat from ** to the corner, ending with ch5, sc in the corner. Repeat this side for 3 sides. Do pretty much the same thing on the 4th side, EXCEPT: instead of ch5, do (ch2, sc in middle of corresponding loop on opposite side, ch2).
Then finish off.
For a dressier look, try using a smaller yarn/hook, like fingering weight (baby or sock weight)and a size E hook or so. You would still be making a 6-inch square (maybe 6-1/2 inches) then edging it. The thing is that the holes created at the end by joining the shell stitches will be smaller.