Some time ago I purchased a mylar bag from Emergency Essentials. These bags are intended to be used inside plastic buckets for storing grains and things. But I'd heard that you can seal them with a regular iron--like for ironing clothes. So I decided to give it a try.
The bag is (well, it was) 20" X 30". I wanted by packets to be 4" X 6", so I cut a 6" strip off the top of the bag.
I'm actually quite pleased with the way they turned out.