I am modelling a watertight container which I intend to 3D print. However, I have an issue: I have the exterior surface and the interior surface created, but the space between these is empty. How can I create a solid to create a wall thickness for the container?
You don’t need to, you already have. If you have an inside wall and an outside wall and they are connected with no holes, the your object will be considered “solid” and your 3D printer will interpret the space between the inner and outer wals as the place to print material.
Virtually all CAD-based models are the same (except maybe voxel models, pretty rare) - there is no “stuff” inside, any enclosed volume is considered “solid”.