[Christ]

How much do you care about aesthetics? If there are stake pockets in your bed sides, you can easily make a tube frame from aluminum of galv steel tubing and stretch thin canvas or something similar over it to get a smooth taper from cab extension to just above the trailer height, and it would be removable for when you're not hauling the trailer (if ever).

Inversely, you could make another one that tapers down gently from the cab extension to the tailgate, for when you're not towing the trailer, and just have a pair of cloth caps that you can easily swap out depending on your needs that day.

The spoiler will help as well, if you just want a regular cap on the back, but placement and angle will be key. If you require the flow to make a steep transition from the cap to the top of the trailer, it will not do much for you. If you move the spoiler too far forward, you will lose some of the benefit.