Step 1: Insert a box dimensioned 6.5"x0.5"x1.25".

Step 2: Duplicate the box and move it to the right ".

Step 3: Insert a box to connect the two walls. Insert a box hole in the middle of the two side walls that is 0.3" wide. Dimension the two walls to a height of 0.25". Group everything together. 

Step 4: Make a partition that is 0.25", 3.45", and 8", with a lip to sit on top of the cover. 

Step 5: Rotate the cover so that it is sitting on the top face. Insert a box that is 3.5"x0.5"x6.5" to allow it to be screwed into the surrounding wall. 

Step 6: Add screw holes to allow drilling into the wall and group everything together.