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

Chained 28 picked up loops in back hump of chain from first chain chain from hook

Trying the Modified Simple Stitch.

Attached front to back (longer side) with SC in each stitch. Did 3 SC in corners. Slip stitched along opening of front piece. 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.

End with multi chain loop for hanging.