What is the Shell Tool?
You can create hollow bodies with the Shell Tool. Basically, you remove a face of a body and standardize the wall thickness.
Using the MAIN TOOLBAR select MODIFY then SHELL
or you can also do CRTL J to open the SHELL TOOL
Select the top of the box you created
Using the PRECISION TAB hollow out the shape with a wall thickness of 2mm, make sure the direction is INSIDE
BEFORE PROCEEDING SAVE YOUR PROGRESS!
Go to the APPLICATION MENU in the top left corner, then select SAVE and SAVE TO COMPUTER