If your sister can give you one that has seen better days as a pattern, it would help you see if it needs any special features.
I think the hospital is getting robbed by this supplier...but without seeing one can't tell.
I think that a good quality flannel or quality cotton is your best bet...don't use anything but quilt shop quality on these...you need the better weave of the fabric. You may need to test for colorfast with her wanting a dark fabric as the bottom. I don't think it is necessary if you figure 2 layers of fabric plus a batting should block out the light. Maybe if you use "soft and Bright" poly batting...it is pretty dense.