Stage One
Start wit a piece of MDF:  9mm … 20" x 36"

Layout
Once you have your board cut to size create your basic sections,,    I have made mine as follows:-
start 3 1/2" inches in from left side using 3/4" strip wood 14 1/2" deep.
section one = 12"  Section two =7"  Section three = 6"
To be honest in hindsight I would have made section two and especially three much bigger.