I will test air curtains in my ID3 project. There was holes in the inner liner, but no holes in bumber to bring any air to those holes. I opened bumber holes with box knife
Built inner panel inside the bumber to direct air smoothly to those holes.I first test like that. If it seems to bring gains I will built littlebit better tunnel where no air cant escape.
I was not able to test yet due to bad weather today. On test I will block the holes on half of the runs.