First I want to admit that I love couscous, and even eat cold couscous at home (home recipes are a bit different from trail recipes though).
The next is that the most ultralight you can go with cold soak containers is not to take any at all. The idea behind the containers is that you put your noodles or couscous in them, add water, put them back in your pack and keep walking for 30 minutes or more, then have the food on the trail. This is a great idea if you are walking from before sunrise through to after sunset, and basically want to eat on the trail.
Yes it is possible to go no prep. There are lots of things you can put on crackers, flat breads/tacos etc to make a substantive meal. But what I am suggesting is that if you are not cold soaking on the move, you don't even need a cold soaking container.
For example
- Couscous comes in foil containers. You can add in water, fold down the top, even wrap a rubber band around the contain and let it sit for 15 or 20 minutes. Then you open it up and then add in meat, beans from the pouches etc and eat from the packet.
- Instant mash potato is ready instantly you add cold water. Just put it in your little plastic packet you would normally use to portion out your food, then add in the water, mix and add in whatever you planned to add to your mash potatoes. Mashed potatoes also come in about 100g packets (are these foil, I am not sure).
- Noodles come in plastic, styrofoam and other containers, which you can use to cold soak. But you can just transfer the noodles to those portion bags and soak in those.
- Instant oats comes in little foil packets as well. Add cold water or milk and it just like eating rolled oats.
If something comes in a foil pack, you can always beg some hot water off a fellow hiker for a warm meal using those packets. But you can buy the proper ziplock bags or silicon reusable storage bags that can be used with hot water.
Like I said, if you are cold soaking while walking on the trail a screw top container is a great idea. But if your hike schedule gives you time to stop and cold soak when not moving, you don't really need one.