[serialk11r]

OK I laid a silhouette against the template and got some approximate measurements for a cardboard box cavity. I also ordered a 3m nylon strap on ebay for 3 dollars (hopefully that and some leftover paint and tape will be all that's needed). The box would collapse vertically for easy stowage inside the car.

I would probably cut a piece of the strap and tie + glue it to the bottom of the box so that the straps pass through the hatch seals and clip together inside the car. The top strap would be anchored towards the middle of the box so that the tension keeps the box unfolded. Since cardboard is pretty soft, the top and bottom will need some stiffening, maybe just some extra cardboard with the corrugation going the appropriate direction.

Bottom edge goes against top of bumper/tailgate, width is just enough to avoid blocking the reverse lights.

Top edge goes out almost to template, makes a 14 degree angle to the spoiler.

Sides make a 17 degree angle to the widest part of the car, which is suboptimal. Angling the sides of the box would make this better but that would make it harder to stow inside the car.

Bottom edge is 21 degrees from the bottom of the car, so probably completely useless as a "diffuser".

Gut feeling this is maybe worth 5% drag or so.