I'm currently using multiple layers of clear packaging tape for my upper grill block. It works great, IMHO. Of course, I haven't taken it off yet, so I don't know what kind of clean up I might run into.

No one has addressed posterboard yet, also known as foam-core. While a great prototyping material (lightweight, easily cuttable, fairly stiff), it won't last against the elements very well. The outer laminations are paper, and will soak through when exposed to water, unless you seal it (most tapes work fine).

Gaffers tape is preferred over duct tape because it can be removed cleanly. It is more expensive, though.

I used duck tape and cut up laundry box on my Honda for a while. Just to make sure that it didn't overheat in the summer. Once that showned that was going to be no problem, I used fiberglass cloth and resin to finalize radiator block, and painted to match. Cardboard or posterboard won't last long if driven in the rain.

I used cardboard as a radiator block. I spraypainted it and it survived a few rains before I was confident that the car wasn't going to overheat.

Duct tape has some limited uses for car aerodynamics
 

Ever watch the "Red Green" show?

Duct tape is fine for "quick and duty" fixes, but it just doesn't seem to hold up well to heavy use. For example, there is actually a fair amount of air flow (wind) that a grill block needs to withstand, and in general duct tape isn't up to the challenge (at least, not for very long).

Also, as another poster mentioned, the duct tape glue can cause problems for some surfaces, so be careful where you attach duct tape. It's probably OK to tape to plastic panels (as I use duct tape for), but you might affect your paint job (or even trigger some rust) if you tape to the car's metal surface.

However, I have still used duct tape in some situations with success. It doesn't hold up very long as a "grill block" (but will work in a pinch), but in some other situations it has less stress on the tape and will work OK. For example, my CRX has many mini plastic covers on its underbelly. While those plastic covers help smooth the air flow some, they still have many bolt holes and other cracks to catch the wind. So to smooth out the underbelly's air flow, I just put sturdy duct tape on those plastic underbelly panels to fill in (smooth the fair flow over) those remaining holes/cracks. Since I'm taping over the plastic panels anyway (not taping to the car's metal), I'm not exactly concerned about the tape glue hurting any surfaces. And if/when I ever do need to remove a panel (to have work done on the car), I'll simply remove the tape (and put fresh tape on later).

I used Gorilla brand tape on cardboard here in Portland, land of rain, and it held up great, until I took it off to use a modified (stretched out) mesh garbage can, so a little air could get through.

Worked pretty good as a hinge for a "gull-wing" door conversion they did on an old Ford Granada...or was it a Fairmont? Been a long time since I saw that one...

Gaffers tape is expensive, but the cleanup is nil. Always a good thing to have around. I would use that, if you coat the whole posterboard surface with gaffers tape you may get more usage out of it during a rainy period.

Re: Anyone tried Poster Board or Duct Tape?
 

Gorilla brand tape is very tough and lasted on my HUGE protege grill(about 14x30 inches) going 80+ just for example. i never got it wet tho. Blue painters tape is great for testing stuff. it doesnt have much stick but it will last a little while at highway speeds and wont gunkify your paint. Clear packing tape makes a nasty mess when you take it off. just my experience!