I used thin macrame thread - similar to a cotton butchers twine/string.

Small Air Plant Hanger - Pattern

Rnd 1: 12dc in magic ring, join, 2ch. (12)
Rnd 2: 2dc in ea st around, join, 2ch. (24)
Rnd 3: (1dc, inc) around, join, 2ch. (36)
Rnd 4: (2dc, inc) around, join, 1ch. (48)
Rnd 5: 6sc, 18sc-dec, 6sc, join, 2ch. (30)
Rnd 6: 5dc, 5hdc, 5sc-dec, 5hdc, 5dc, join.
Make a chain as long as you need for hanging - either a small loop or long chain…create a loop at the top. Leave a long tail to sew back down the chain and secure inside the plant hanger.

We added weight to the hangers using small rocks.
The rocks also help to hold the air plants in place.