I bought white silk fabric (45 inch wide 8mm habatoi) from
Dharma Trading. I bought 6 yards to make sure that I had extra for any mistakes I might make. Then I dyed it using a low water immersion technique with dyes that I had from Dharma. The finished size is just about the same as a twin mattress, and I made a separate pillow case. The elastic is attached about 12 inches from the top and bottom, and I used a 36inch zipper - lightweight invisible on a mesh backing, though I didn't install it as an invisible zipper. The silk is very easy to snip and rip to get nice and even.
This is actually the 3rd silk sack that I've made. The first one I bought a pre-made double silk sack, then dyed it and cut it down to a generous single. It was almost perfect, but not quite, so last year made one from scratch. Both of the first two had the built in pillow cover. I decided that I'd rather have a separate pillow case, and wanted to make some other improvements, so this one is the NEW! IMPROVED! product.