At about 230g for 800ml capacity, the GSI mug is rather heavy. If you're rehydrating meals, you're probably thinking lightweight, so I'd suggest recycled food pots in a huge variety. Or go to your local pound shop for a wide selection of food containers.
Ideally, you'd be looking for something that will nest in your cook pot, and have a rounded corner in the base, to make cleaning easier.
For my MyTiMug cookset, I use a plastic pot that contained cookie cutters, bought in Poundland; the mug fits inside the pot.