:teehee: That is a cute one! Acrylic is fine as long it's a very soft type like Bernat Satin or Caron Simply Soft.
Bamboo is very drapey and since that's not a good thing for a hat it's best in a blend. I haven't personally tested it, but I got this from other folks who have used it. Both cotton and bamboo will have less problems if knit more tightly.