You all have the best ideas!
I asked you what button to use for the cardi and you chose -- no buttons!
So I made ties:
I attached the yarn at the top of the body, right under the motif yoke. I chained 50, made a 3-stitch bobble in the second chain, then slip-stitched back up the chain to the body. I slip-stitched a couple times on the body, then wove in the ends. I made the tie on the other side the same way.
To wear it, I laced the ties upward through the edge loops on the yoke.
It looks good, too, with the ties just tied right at the top of the body, and the edges of the motifs folded back. But I don't have a good picture of that.
(Her Dogginess actually approves, though she looks rather aloof at the moment.)