Hi Frank - shell can be used to change an object to have a certain "wall thickness".

The general way that you use it, is you have a solid object, then you select a face of the object that you wish to have as an opening (you select faces by doing a second click on an object). Then run shell. You can now enter a distance, or you can pick 2 points and the distance between the points will define the distance.

This will create a result like this:


You can also use shell to thicken a surface into a solid, here is an example:


Please let me know if you need more information.

- Michael