I built a loft in the bus, over the cab and extending over the hood (see the floorplan).
Having raised the roof, there was a pretty sizeable gap between it and the windshield. There were a few other factors too:
Building a loft with an angled front surface naturally solves all of these. It isolates the cab to reduce the heat leakage through the windshield, gives me about 40 ft2 of extra floorspace, and helps with my overall wind resistance (yes, it's still abjectly terrible, but it's better than it would be otherwise).
The front of the loft is sloped backwards in order to keep things a little more aerodynamic. The raised roof adds around 20 ft2 to the front facing surface. A little slope won't offset this, but it's better than leaving it flat.
After the picture at right was taken, a sizeable tree branch fell and almost hit the bus. It took out the driver's side mirror and left a gash in that corner.