These joins lie flat.
With a right side facing in front and a wrong side behind I put a half treble in the front square and a half treble in the back. When opened out it becomes a fairly flat seam.
The edge was one round of half trebles then 2 trebles, ch 2, 2 trebles, skip 3 all the way round. The corners were 3 trebles, ch2, 3 trebles.