How to extrude inward to make a rectangular horn antenna?

Question

I only want to draw the head of a rectangular horn antenna as shown below.

My attempt is as follows. I don't know how to extrude inward to make a hollow pyramid.

Steps:

1. Add a cube.
2. Go to Edit mode, press 3 (Face select), select the top surface, press S while dragging mouse and left click to confirm shrinking.
3. Select the bottom surface, press E to extrude, right click to partially cancel the extrusion (it leaves an additional rectangular edge), press S to shrink the additional rectangular edge and left click to confirm the shrinking.
4. I want to extrude inward to create a hollow pyramid. What tools do I have to use?

I am new to Blender and learning at random order.

Answer 1

Option 1 for this: You already made an inner face with I, now select that face, press E, take it up to the place you want:

scale it down with pressing S and follow by pressing Shift+Z(that will lock axis only to X and Y:

This option works if it doesn't have to be 0.001mm precise... If you want it created by a modifier, then remove that inner face and use Solidify modifier with even thickness.

This, however, will create that bottom sharper, but you can easily tweak that