A plastic takeout container has a hard shell and a waterproof lid plus the bonus of clear plastic lids on most. They are called to go containers, microwaveable takeout containers, disposable containers, and deli containers. They are a little less sturdy than an Altoids tin but if they last one trip, that's enough because they only cost a few cents. I've searched through a lot of restaurant supply houses and they all have perfectly waterproof (water resistant) plastic takeout boxes but none that are the right size.

I did some calculations - An altoids tin is about 4 ounces in size and the Maxpedition pouch will hold up to about an 8 ounce container - if it's the right shape. So far I haven't been able to find any plastic takeout containers or disposable containers that are in the 4 to 8 ounce range and have acceptable dimensions.