Seaming Baby Bootees

I’m trying to use up odds and ends of baby yarn by making booties to donate. The pattern I’m using draws together the cast-on edge of garter stitching to form the bottom of the bootie. It’s a cable cast-on. Can anyone advise the best method to use for the softest seam? I’m thinking mattress won’t work in this particular case with the cast-on edge??? Back stitch? Whip stitch? Done right sides together? Thanks so much.
Here’s the pattern:

I just tried sewing a cable stitch cast on together in this situation and I used a whip stitch drawing together the edge of each cable cast on and it seems to make a pretty nice seam. I sewed it from the outside and the inside feels very flat. I guess you’d say I butted the two sides of the cast on edge together.

Thank you so much, Merigold. I’ll try that today.