I've made bags for insulating zip lock bags a few ways but the one I like the most are these. They use some left over momentum 90 along with an insulated fabric called Insul-Bright. which is sold on-line or in some sewing shops like Jo Ann's fabric. The manufacture has this to say:

Insul-Bright consists of hollow, polyester fibers needlepunched through Mylar. The hollow fibers resist conduction, while the reflective mylar resists radiant energy. The energy, hot or cold, is reflected back to its source. Easy to sew, plus your finished project can go right into the washer and dryer.

It's very easy to sew and quite easy to pack.

JFF