What you could do is use a Level and tape to make sure that the Lines and The "Box" it is in are straight and even. use the Level to determine where the tape would go. apply tape and put inside the center of the box. I just eyeballed Mine though and it came out great..
SharkFinAntenna.com has added the 2008 Scion xB Shark Fin antennas to their "Order Now" page... Select the quantity, make, model, style, year and paint code.
Unit price is $69 + tax & s/h ($12 USD in N.America)
What will you receive?
* VG Shark Fin Antenna paint matched to the color code of your car
* Pre-installed 3M custom double-sided adhesive gasket
* Pre-installed Fractal antenna with coiled cord attachment
* M5 & M6 attachment screws
The link in the OP shows it for a few dollars more, and it looks like they're just a drop shipper, so if you go directly through Visual Garage for the VG Shark Fin Antenna, you'll save some money.
So does the shark fin do a better job of pulling in weak stations? I noticed that the basic Pioneer unit in my '08 does a good job of locking in on stations. Sometimes it's too good. The scan feature takes forever to make a loop and we only have a dozen stations in the whole region.
Honestly, I don't know... There are only a few stations I listen to and they all come in just as well as they did w/ the stock antenna. I could test range, but since I didn't do that before, I wouldn't have anything to compare it too ;(