best way to join center rear

[Pleater]

I have access to tape and ribbon and have used a simple seam, pressed the allowance open from the inside and then sewn a ribbon onto the inside, wide enough to cover the raw edges.
The ribbon was sewn on each side of the join, quite close, from the outside and then from the inside at the edge of the ribbon.
As long as the thread is well matched I have a wide selection of colours, and is not pulled tight or running loose, the join is not very obvious at all. I did a few by hand at first, and then used the sewing machine when in a hurry, and never did another by hand.

My sewing machine and I are, however, very old acquaintances so a bit of practice might be required to get the result just right if yours is a recent acquisition.