>If the bags go inside, why would you need clips?
>Why not use something simpler like a padded Tyvek mailing envelope?
Excellent question Michael. Here are three photos of the cozy that hopefully will help you see how it is made. If folks can come up with even better, easier, cheaper cozies, that would be wonderful. I started out with a simple envelope design, but after watching the boys try out a couple Philmont practice meals, the design evolved.

The key cozy design parameters are:
1. Dimensions of the Philmont supplied foil food pouches. An example is shown in the first figure. I bought 6 different meals, containing 23 foil pouches which varied from 7"x10" to 4"x6". One edge was always between 6-7" so I made the cozy 7" wide. I made the cozy short enough so the pouch would stick out the top to make pouring in the boiling water easier. I made the sides 3". I added clips as an accessory when I saw the boys let the top edge of a 4"x6" pouch of corn slip down into the cozy while practicing pouring in boiling water. Now clips hold the short pouches so their top doesn't slip down inside the cozy where their contents may spill.
2. Volume of the cozy needs to be large enough to hold the maximum volume of water in a single serving of any Philmont meal. That turns out to be 12oz (1.5 cups) plus the volume
of the freeze dried food already in the bag. To address this I made the cozy 3" wide at the bottom, giving it the volume of a triangular prism 7" long.
3. Very desirable to have the cozy stand up by itself. The food needs to rehydrate for 10-15 minutes after stirring. I saw the boys trying to sit the cozies down during this time. Envelopes don't work very well for this feature. The triangular shape allows the cozy to hold the pouch upright and being widest at the bottom the water weight and hence the center of gravity is low on this design making the cozy harder to knock over while rehydrating.
4. While not a required feature, I wanted the cozy to be as light as possible so I used 7"x9" piece of 2mm art foam and folded it as shown in the photo so that it would pack flat. The cozy weighed 5 grams. It worked great holding the pouch. It was plenty big to hold the volume of water. But I found that when BOILING water was poured in, and allowed to sit for a few minutes, the foam properties changed. It no longer held its triangular shape. Increasing the chance that the cozy would fall over and spill the food while rehydrating. My quick fix, one week before we left for Philmont, was to slip in a thin sheet of plastic on the two sides of the cozy. This worked very well at holding the cozy shape when hot. It also pretty much eliminated the need for the clips, but we carried and used the clips at Philmont to just be EXTRA safe not to spill any food using this new cooking method.