Here is a stash-buster project to crochet a cozy for a 500 ml water bottle.
Materials I used:
E (3.5mm) hook
kitchen cotton worsted-weight yarn (I used Lily Sugar'n Cream for the one pictured here), 6 colorways: main color (MC) plus 5 contrasting colors
This pattern assumes you know how to chain (ch); slip st (sl st); single crochet (sc); work in a spiral without joining; use stitch markers to mark beginning of round (rnd).
Working in spiral rounds throughout, work Rnds 1 - 6 in MC, then change colors every 2 rounds. I worked the last round in MC to have it match the base.
Rnd 1: 8 sc in 2nd ch from hook (or use magic ring). (8)
Rnd 2: 2 sc in each sc around. (16)
Rnd 3: (2 sc in next sc, 1 sc in next sc); repeat around. (24)
Rnd 4: (1 sc in next sc, 2 sc in next sc, 1 sc in next sc); repeat around. (32)
Now begin working up the sides. After a few rounds, place water bottle inside to test for fit and rip back past rnd 5 to add or remove another row or 2 to the base before forming sides.
Rnd 5: sc around in back loop only. (32)
Rnds 6 - 24: sc around (mark last st). (32)
Now form the strap.
Rnd 25: sc in next 16 sc, ch 150 (or preferred number) for strap (I estimated my preferred length to need 150 ch sts, allowing for some stretch once finished), sl st to last st of previous round, sc back across ch, continue to sc along rest of rnd, sl st to 1st st of rnd, fasten off. Weave in ends.