Been out of town for a few days, but I'm back to drawing. Not sure yet where I want to fall in terms of compromises between absolute width (e.g. 48" plywood sheets), internal area, aero impact and turning radius, but having the box on top be replaceable means I can build several tops for different uses.
The Insight has about 28,150 cubic inches of cargo space in the hatch + hidden cargo area under the floor, or 16.3 cubic feet.
If I were to pick up a stock HF trailer with a platform of 40x48" I would have a maximum angle between trailer and car of ~82.5° with the trailer sitting 30" behind the ball. Internal area at 15" uniform height is 28,800CI, or 25,920CI if I taper it to 12" at the rear.
Widening the trailer to 48" would not affect turning much - max angle would still be ~80°. Almost all of the trailer would still be out of the windstream, with only the top corners poking out, though Cd would be increased compared to a narrower trailer. I'd have roughly 34,100CI of internal volume assuming a uniform 15" height, and 30,700CI with a taper, both accounting for recessing the wheel wells to fit the wider box. Alternatively, I could look for another trailer that already comes 48" wide, so I wouldn't have recessed wheel wells, which would give an extra ~400CI of volume.
Extending the trailer platform forward 6" and leaving it at 40" wide reduces angle before the trailer contacts the car to approx. 75°, increases the height at the front of the box to ~17" while not hurting aero and total volume ends up at ~32,880CI/30,000CI.
Extending the trailer platform forward 12" reduces the turning angle to around 55°, increases the height at the front to 19" and increases total volume to ~37440CI/34560CI.
Attaching a 20" hollow semicircular nose to the front of the trailer (as it looks like aerohead did in his CRX trailer) would give a maximum angle of ~60°, allow a maximum height at the front of the trailer to be 22", and give an internal volume of approx. 40,800CI/37,920CI. The tip of the cargo box would be 10" behind the ball.
Attaching a 20" hollow semicircular nose to the trailer and setting it about 6 inches back from the front of the platform would allow for a maximum turning angle of ~80°, a height of 20" at the front and a total volume of approximately 36,700CI / 34,180CI. The tip of the cargo box would be 16" behind the ball.
It looks like a rounded nose is the way to go for maximizing internal volume while maintaining turning angle, though it does increase the difficulty of building the trailer unless I can find something prefabricated that I can adapt.
Please excuse the slightly sloppy drawings. ^^