Ive just been to Aldi & found a great product called a "Pour & Store" bag.
These are 240mm x 155mm size heavy-duty ziplock bags with 1 litre capacity that are designed to hold left over cooking sauces, oils etc & even left over meals.
Theyd make pretty good containers for FAKs & could be used as subload pouches for other kit.

The food grade plastic that theyre made of feels almost as tough as a platypus bladder & theyre dirt cheap, too.

I paid 89 pence for 8.

The product is the Poly-Lina Pour & Store