I joined the Bloominglabs Pumpkin Launch team, and after learning that we would need to make the sling for the trebuchet, I checked Ravelry to see if anyone else had made one. Rav did not let me down; I’m following arcadienne’s excellent notes. 
The photos are the amount of cord we had on-hand, used for proof of concept. I think I need a slicker and/or stretchier cord, and I might need to cast on a few more stitches.
09-21-2016
Sling #1 (white):
100ft solid braid polyester rope
US 17 needles
CO 5sts.
K one row.
Next 4 rows: k1, yo, k to last st, yo, k1. (13sts)
K every row until almost out of rope (~14 ridges).
Next 4 rows: k1, ssk, k to last 3sts, k2tog, k1. (5sts)
K one row.
Bind off.
14” x 7”
Needs to be longer and wider.
10-12-2016
Sling #2
3/16” diamond braid poly rope
US 17 needles
CO 7sts.
K one row.
Increases:
Row 1: k2, yo, k to last 2 sts, yo, k2.
Row 2: k2, ktbl, k to last 3 sts, ktbl, k2.
Work Inc Rows 1-2 five times total. (17sts)
OMG, this is killing my hands, and the fabric is way too tight. It might be better on 19s, but I think I’ll just use thinner cord instead.
10-13-2016
Sling #3
thinner rope
US 17 needles
CO 5sts
K one row.
Increases:
Row 1: k2, yo, k to last 2 sts, yo, k2.
Row 2: k2, ktbl, k to last 3 sts, ktbl, k2.
Work Inc Rows 1-2 thirteen times total. (31sts)
Knit every row for a while, ~6” or length to match pumpkin diameter x2. Attach rope as needed using magic knot.
Decreases:
Row 1: k2, ssk, k to last 4 sts, k2tog, k2.
Row 2: k all sts.
Work Dec Rows 1-2 thirteen times total. 5sts rem.
Bind off: k2togtbl, *sl st back to left needle, k2togtbl* to end. Leave a long tail and weave it in thoroughly.
10-18-2016
Sling #4
mason line (#36), 230yds/cone; one whole cone + a bit from a second cone.
US 17 needles
CO 5sts
K one row.
Increases:
Row 1: k2, yo, k to last 2 sts, yo, k2.
Row 2: k2, ktbl, k to last 3 sts, ktbl, k2.
Rows 3-4: k all sts.
Work Inc Rows 1-4 thirteen times total. (31sts)
Knit every row for a while, ~6” or length to match pumpkin diameter x2. Attach rope as needed using magic knot.
Decreases:
Row 1: k2, ssk, k to last 4 sts, k2tog, k2.
Rows 2-4: k all sts.
Work Dec Rows 1-4 thirteen times total. 5sts rem.
Bind off: k2togtbl, sl st back to left needle, k2togtbl to end. Leave a long tail and weave it in thoroughly.