Cast on 60 st and join in the round.

Knit mistake rib (as in the following two rows) until long enough:
a: (K2, P2) all round.
b: (K1, P2, K1) all around.

K all around one round once long enough.

Decreases to form the base:
1: (K8, K2tog) all around. (54 st)
2: (K7, K2tog) all around. (48 st)
3: (K6, K2tog) all around. (42 st)
4: (K5, K2tog) all around. (36 st)
5: (K4, K2tog) all around. (30 st)
6: (K3, K2tog) all around. (24 st)
7: (K2, K2tog) all around. (18 st)
8: (K1, K2tog) all around. (12 st)
9: K2tog all around. (6 st)

Cut/break yarn leaving a long end and thread it through the remaining 6 st. Draw this loop/hole closed and sew in the ends.

I will probably apply an i-cord edge to the top/start edge maybe with a shoulder strap for carrying the bottle.