I started with plywood... then went to "boatboard" which is black plastic often used in boats... now with thick aluminum... there is signboard material that would work well - honeycomb structure plastic with thin alum sheet on both sides, that is probably a good compromise of weight, stiffness and price... I am planning to use this material for an undertray
I use a flat aluminum bar at the "rear" of the splitter and towards the front have a custom bracket where the splitter is attached on both sides to the bracket via quick release pins. Then in front of the car I have two threaded rods I can use to adjust the angle, those rods are welded to the alum bumper beam
There are pics somewhere in my journal
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