Darn easy to do. Just be careful with the heat. You don't need the apex of the flame tip but the outer light part of the flame. It's a gradual heating process. Too much heat will make it brittle. You can do a grill blocker without a workbench. Attach one side (top or bottom) to where it's going to be and gradually heat the stuff applying pressure to form it to the shape. Finish attaching and you're good to go!

Another thought (GM actually came up with this) is that you don't really need to totally block the grill off. A lot of Dodge Dakota guys use (honest to God) metal gutter guard as a grill guard. Rocks can't get through those little holes. This also blocks air. That's why GM SUVs and trucks have those metal inserts in them now. They did a test in 2000 on a Silverado with cardboard taped on it including blocking between the body and frame.

What do you think has the least effect on temp - blocking the upper or lower grill? (Like the ones on this Focus)

Very professional looking. And cool looking too !
It looks like something that you would see done at a speed shop.
Nice touch having the aluminum which matches the car.

Have you noticed any MPG gains ( lazy. didn't read entire thread )

Thanks. I've change my lower grill block a little bit. Now the lower grill is covered 50% for better cooling. I have noticed 3 more MPG on highway with that modification. I'm planinng to make rear wheel skirt pretty soon and see what kind of gains I will get