Hi Blzzdqueen I used to make them a long time ago. Mine are pretty simple.
I have a pattern I can send to you. I don't have it written down but I will jot it down and send it if you want it.
Basically you just cut one long piece and then add pockets. Make two sides (with pockets) and inset the sides. Pretty simple.
I think you can buy some plastic like raincoats that could be used for the cold packs.