If you really want to test the idea cheaply before buying the straps, I’d suggest a length of chain from the hardware store instead of a piece of sheet metal. If the metal bends away from the road even a little, it will lose contact and the test won’t work.
(I think the rubber strap is a better long term solution than chain, unless you want your car to sound like Marley’s ghost. But the stuff about static buildup causing sleepiness and motion sickness is pure bolognium.)