So, I made another one.
This is in an 8ply like the previous two, but it's a stiffer yarn than the previous slippers.
AND, I used a 4mm hook, not a 5mm hook. As you can see, it fits my foot MUCH better.
It's much more snug. Not sure how well it'll go over socks for winter, but it's a much better spring/summer alternative.
I didn't make the string across it quite long enough though. Bummer!
The buttons are also special: I bought them in Canada years ago, they look like wood, and these are the first thing I've ever sewn them to )