PB, you could do just a gathered shoulder, but it would be much bulkier. As for using a thick layer and a thin layer of fabric, you can do double layered slings no problems.

The biggest concern with lighter fabrics is that your seams won't be strong enough - because most quilter's cottons (for example) are just not designed for the type of stress. The concern is that even with reinforced seams, the fabric may tear at the seams because it is too light. But doubling will help reinforce it.