I made a few with shell borders and a few without.

I made the back square one row longer than the front square and it needs to be 29 or 35 stitches wide and 29 or 35 rows long for the shell border to work without making adjustments.

Brown & Red potholders: Chained 28 picked up loops in back hump of chain from first chain.

Attached front to back (longer side) with SC in each stitch. Did 3 SC in corners. Changed to contrasting yarn for shell border.

Shell Border: 7 DC in corners, # skip 2 SC, SL, skip 2 SC, 5 SC in next stitch, repeat from # to corner.

Blue & White and Blue & Yellow made with 5.5 mm hook.