Intermission: Take a look at that pan. I have no idea what it is really called but looks like a cake pan to me. It comes down and then the lower edge curves in and it rests on those I-Beams .. or does it? Not directly. There is something between the I-Beams and the pan. Could it be boards? Maybe. That part is up to the modeler to determine I think. In any case I am going to say that “THAT” .. is the bottom of the pan.
If you clicked off the model and that surface is no longer selected then click again to select. Now .. we are going to use a new too. Click “P” or select the Push/Pull Tool. We are going to PULL that surface down to those boards to create the vertical dimension of the pan.
Hint: Since we want to pull this surface vertically let’s LOCK the cursor movement in that direction. To lock the cursor movement in the Z-axis (vertical/blue) hold the down-arrow key while performing other operations. Cool huh?
With the Push/Pull Tool selected, hold the down-arrow key and pull downward. The surface will extrude. Pull down and Push up until the bottom of the extrusion aligns with the top of those (supposed) boards. Then let go.
Save the file. Ok. I will stop telling you to do that but .. you get the idea.