I don't know about the '08, but I use a locking gas cap for my 2007 H2. The cap states that it is made for an H2 thru 2006 model year, but it fits the 2007 perfectly. I have been using it for a year, through the hot Las Vegas summer.
I found it at Checkers / partsamerica.com
It is listed as the Stant 11511
http://www.partsamerica.com/ProductD...rtNumber=11511