I don't have a Pet Fly's sling, but I just bought a cloth cross-body sling from Old Navy and have been using it for Pixie. She's 4.5 lbs and has more than enough room. It is pretty big so I just push the excess fabric down once she's in so she can see out of it. She likes to have her one leg dangling outside of it. It's only $12.50!
I didn't want to spend a lot of money because I wasn't sure if she'd like it. It's very comfy to wear.
http://www.oldnavy.com/browse/produc...scid=555510032