At that weight, I'd avoid wool, too. The stretch is the issue. Crocheted, it probably wouldn't stretch so much, but what you really want is something with diagonal stretch, but no stretch in the other directions. That way you've got a bit of give for comfort, support and snugness, but without the danger that baby could fall out. You'd also have a hard time threading wool through ring slings, I'd think.

I was actually talking to someone recently who experimented with wool slings. They were great for little babies, but quickly stretched too much and were unsafe.

If you're after cheaper options, you could find some bottom-weight fabric from spotlight or somesuch, with some diagonal stretch, and sew that up.

2nd ring slings are not that expensive, if you have a look around. They are perfect for hip carries, yes.