I searched on ebay and there are over 800 hits for "Cell Phone Antenna Booster"

Then I found this site:

http://www.andybrain.com/extras/cellphone-antenna-booster-sticker-review-and-test.htm#why

that basically says that the stick on antenna boosters do not work.

That's kind of what I figured.

But it does seem possible to me to make something that would boost the signal for a cell phone, but their idea of making an antenna that is the exact length sounds like a lot of trouble.