This thread mentions blocking dust inflow into the cab. It is my opinion is that a full or partial "window" does not really help much. It creates a strong low pressure pocket behind that windshield from the air flow coming over the top of the roof. This dirty air pocket then swirls in from the sides and through the gap between the bed front wall and the cab. So, it can actually aggravate the dust in the cab, especially if you have a windshield.
The route I took was to create a porous windscreen behind the cab. In principle it is like a giant version of the rear windscreen that you find on many convertibles in the automotive world. The materials that I used were canvas and super heavy duty window screen fabric (rubber coated fiberglass mesh). Home Depot sells this as "dog proof" window screening. Clean air is entrained at the rear of the roof and is sucked down into a low pressure area over the bed. It then bleeds through the screening and spoils the low pressure pocket behind the bed. The rear is tied down to the tail gate with 3 ball bungee cords that pass through grommets in the windscreen and then through loops attached to the tail gate. This makes it quite easy to access the stuff in the bed when you need to. You still can see out the back due to it being a window screen. It blocks less than 50% of the light in spite of being such a heavy duty material.
Food for thought for any one that is really irritated with the dust devil behind you....
Rear view:
Bikini top, Rear Diagonal view, also shows tan fabric dust barrier between bed front wall and cab:
Bikini top, side up view:
Bikini top, air scoop & gutter, also shows snap fastener attachment of windscreen to upper crossbar (bent electrical conduit):
Side view, air flow diagram: