I knit a thumb hole! It was hard and I ripped portions of this hand warmer out several times prior to being happy with it, but I successfully knit a hand warmer with a thumb hole.
I used the tutorial on Purl Bee for the Long Striped Hand Warmers, but instead of two colors I used three. I also didn’t make the write as long as the tutorial (mine was six inches rather than the eight recommended). I was slightly scared to use the stitch holder and then have to pick up the stitches later.
I printed the directions and kept them in the bag with my yarn, which worked out great. I didn’t have the energy to start the next one, but as the bf pointed out, I have a while until it is needed.