I completed the major portion of the upper desk cabinet yesterday. This morning my wife and I brought the monster into the house and placed it on the desk. we did this at this stage mostly to insure that the unit would actually negotiate the double doors entering into the office. I wanted to do this before proceeding and finding out it just would not fit. ;~)
The design strongly relies on the strength of the 4, 3 1/2" oak rails that span the 90+ inches. The method used to test for SAG shown below is tried and true for me. I believe that there should be no noticeable future sag.
Below is the test fit on top of the desk. I still have quite a bit to do, add doors, cabinet backs, varnish on the outer surface of the front and back face frame, add feet to secure the unit to the desk top, a cubby unit to house 9 in/out drawers in the middle, the center arch trim to match the doors, and the top trim.
The plans.