Loved the Eco Duo. I would change the envelope opening to have the first inch of the cast on stitches worked in seed stitch (or maybe rib) to prevent rolling. Especially with a hot water bottle that has the little tab with a hole at the bottom, the envelope opening has a tendency to gape.

Used Lucy Neatby’s Modified Conventional Bind Off for the turtleneck edge. Very tidy.

Further edited--I re-seamed the envelope edge to draw it up a little more. If making again, I would go down a needle size for the ribbing, or possibly use a slipped stitch rib (as with a sock heel flap).