Another method I used was offcuts of black HIPS ( high impact polystyrene) about 1mm thick, laid it on the inside of the grill and heated it with a heat gun until it sank slightly into the honey comb. Then a couple of dabs of impact adhesive and it stayed there.
You could also use any other thermoplastic that'll soften with heat.
I got an extra 20 miles per tank iirc, got my first 400 mile tank on the 1.6 petrol using that grill.