I live in Canada, it's winter now and I don't want to spend too much time outside.
Here's my fast, cheap & easy version.
Used pipe insulation at Home Depot, only $3 for a 6ft (2m) tube.
Cut the tube in two 3 ft pieces and nudged them in between the horizontal grille bars and pushed both ends through the last left & right openings. The foam tube ends extend maybe 6" inside the bumper but do not touch the radiator. Didn't use any glue, tape or zip-ties. Almost a week later they are still in place.
Here's a front shot:
And a close-up:
There's a small area below the license plate where the foam allows some air, so approximately 90-95% of the lower grille opening is blocked.
When spring/summer come, I will probably try another grille block.