Make an open box with one prim?

I'm new to building, and was shown how to make a cube hollow today. When doing so, it opens up a hole through the cube. If I wanted to make a box with just one side gone, I would have to use another prim to make the 'bottom' after hollowing it out. I am wondering if there is a way to move the 'hollow' so that it carves a hole out of instead of through the cube, saving me a prim.

Nope. Not unless you're willing to live with a tapered cube. You can close one side by taper but you get a triangular cross-section.

That's what I needed to know! Thanks for your help, Aodhan. If it was not for the help of people here and in SL, my boxes would be using five prims. Would the ability to offset the hollow part (thus saving on prims) be a logical feature, or would there be drawbacks to this if implemented?

My guess? They would at least have to redefine how the cube is made. As it stands you can think of a cube as having the structure of a squarish ring. They would have to change that structure to be able to make a 5-faced cube.

In all likelihood, if ever we will see it, it will be a new prim type just like the cylinder and the tube are not the same structurally but sometimes can assume the same shapes. The cylinder structurally is effectively an extrusion and the tube is a solid of revolution.