[Big Dave]

Ever thought of replacing the moveable part of the lid with a roll-up tambor tonneau - something like a ReTrax tonneau?

You would gain ease of conversion for max utility. Simply roll up the "tonneau" part and drive the four-wheeler or yard tractor into the bed. When you don't need to accommodate the bulky freight (99% of the time) you roll the "tonneau" down and enjoy the increased MPG.

Of course, there is no free lunch. Using a ReTrax would be uber-flexible and very sturdy but you'd lose the center window altogether.