Had several attempts to figure out sizing and a good caston for the additional stitches for the body.
Final version cast on 10 stitches, increase to 20 stitches.
Buttonhole row: sp1, k7, k2tog, yo x2, k2tog, k8
Flap length about 3 inches
Added 22 stitches for a 42 stitch tube for the body. About 4-6 rows of garter (one attempt, the garter section was too long for the buttonhole to be secured in the stockinette)
Body length is about 5.25 inches. I think it would have been nice with a decrease row (or two?) before the kitchener finish.