Doesn't Jen (mom-and-me) make the slings out of hemp fleece? I know it seems like a picky detail, but I've worked with hemp fleece and it doesn't seem like it'd be as stretchy as jersey (think - sweatshirt vs. t-shirt here), yet it is still very stretchy! You cannot make a tube style sling out of something that stretches more than say 3 inches across without it having to be adjustable. I tried making non-adjustable pouches out of hemp fleece WAAY back in the early days and was quite unsuccessful.
Now hemp twill and muslin make great pouches.
Now hemp twill and muslin make great pouches.