You can use any Bulky weight yarn for these.

My gauge is between 3 and 3.5 stitches to the inch using a thick and thin yarn.

12/13/20, pdf has not been updated yet:
There is a problem with the decreases. At that point you have 25 stitches.

The decreases should be:

k2tog, k10, k2tog, k11
k2tog, k9, k2tog, k10
k2tog, k8, k2tog, k9
k2tog, k7, k2tog, k6, k2tog

Alternatively, when you rejoin in the round k2tog (to close the gap). Then continue as written.