First, the neck casing: Nothing complicated, just a casing with a drawstring. The purpose of the button is that you won't have a zipper to keep the quilt around your shoulders. The button does that. You could use a snap, but buttons are more reliable and easier to install properly. Don't use elastic here.
Second, the elastic straps *should not* be numerous or complicated. Just one or two (I use two) straps, evenly spaced. More than two and you will look like a clown trying to thread the sleeping pad between the quilt and a bunch of cords.
My straps are just plain, flat, 3/8" elastic from the neighborhood fabric store. They are adjusted by running them through 3 tight holes poked in ovals of milk carton plastic. That is a simple, ultralight tensioner. They unhook from one side and are held with a Velcro sandwich -- two 1 1/2X3/4 strips of hook Velcro stitched together, face to face are stitched to one side of the quilt. The end of the elastic is stitched to two 1X3/4 loop Velcro strips stitched together with the loops facing out. This is a secure fastener that doesn't expose hook Velcro.