I once did a home-made cover for my NW single CAI from foam-backed foil and some foil tape I got from Home Depot for insulating pipes. It did well in the IAT department, but simply looked like sh*t.
During the summer here, I see 14 - 20 degrees over ambient IAT's, with the insulation I was seeing 6 - 9 degrees for 30 minutes or so, then it would eventually heat soak and I'd see 10 - 14 degrees difference.

I installed a large off brand filter, cone shaped with reverse cone at the small end, not sure what the size is, but MUCH larger than the K&N I have from NW.