[RedDevil]

The skirts need to be really stiff to work right.
The cardboard does not look very rigid, esp. those 2 vertical folds. If it deforms or vibrates that will reduce its effect big time.

Cardboard is usually much stiffer in one direction than the other.
The central layer forms a wave shape between the outer layers, creating channels in between. It bends easily parallel to those channels, not so across them.
I suspect you used it in vertical orientation (the folds seem smooth, like they'd be when folded parallel with the channels)

If you do a rerun with horizontally oriented cardboard skirts that may show better results. Make it a double layer just to be sure.

Even if it works I'd expect a reduction in fuel consumption of about 1 - 1.5 %.
It will be hard to prove it does make a difference.