How can I close this?

one way is to simply put a flat bottom on it, and fillet it, inside and out.

another thing you can do would be to make the main box, shell it, then add the lip as a solid chunk, and shell that from underneath the lip

or you could make the lip and the main box as separate bodies, shell them, then merge.

I think trying to do what you have set up right now would not be successful.

Copied the dimensions from your sketches, made a move face, made a bottom from a convert entities-sketch on the resulting end-surface (so that the depth can easily be changed) extruded inwards, added two constant radius-fillets.