Remove the top that was being test fit in the last step. Cut the bottom of the desk out of 3/4 MDF. test fit the bottom of the desk in the closet. It should fit against the bottom of the strip of MDF installed in the closet.
Cut interior beams from MDF and test fit them on the bottom of the desk as shown. These interior pieces will be sandwinched between the top and bottom of the desk to form drawers, cable trays, and tool wells.
Attach interior pieces with wood glue and countersunk screws from the bottom. Make sure you drill holes prior to inserting screws to avoid splitting the wood.