Using medium gauge clear PVC sheeting, and a simple women's hair straightener, you can create sealed packages for storing items.
In the photo I have some backup fire starting materials I keep in my bag.
I've sealed a lot of shit with this method though. Resealed some stripped MRE's and added my own contents to make custom MRE's. The seal is strong enough to use as a makeshift flotation device if you sealed air. I try to remove the air to save space. But I also have some water packets that I made by sealing ice cubes with this method.
I've sealed electronics, gun parts, 1st aid items, etc..
Once you open the package in the field you can't reseal it though. So you want to plan it out and not seal a bunch of items that you aren't gonna use after you open it.
I set the hair straightener to it's lowest setting and press the edges of the plastic between it for barely a second. If you press it together too long it warps and melts the plastic. When done properly, the edges are perfectly straight and take on the texture of the surface of the hair straighteners hot plates. You can repeat the heat application once it cools for added strength. just not too much heat or it destroys it.
This also works very well with empty pouches from rolling tobacco. Like a "Tops" pouch, which is like a mylar plastic that has a greyish inner film. That plastic material is made for heat sealing. You might be able to buy it in bulk even. I'm not sure the type of plastic that it is or i'd buy some.