Air dam time
I cut my brackets from Alu paver edging. It's light and sturdy, rust free, easy to work, is already broken up into segments and half the brackets are even pre-drilled.
I cut it with a regular wood working blade. I've used fancy TCT blades in the past but don't think they're worth the expense and hassle of swapping blades.
I used bolts, washers and lock nuts to secure these to factory holes in the bottom of my bumper cover (the holes are there for a factory option body kit).
The air dam itself is 75mm (?) plastic lawn edging that I recycled off my old previous Renault. Attached to the brackets with galvanized, self drilling 'stitching' screws.
This car is essentially a sedan with a 1" suspension lift, so there's plenty of air getting under the car. The airdam has had a noticeable effect on coast distances and noise. Really makes you wonder why the OEM's didn't do something so simple in those days.