Why not build your own? This link is a good start: http://eplaya.burningman.com/viewtopic.php?t=33842
I liked his idea but altered a bit for my taste by:
Using a 3.5 gallon top bucket instead of a 5 gallon.
Siliconed the 3.5 to the 5 gallon (bottom bucket) lid.
Drill holes through both lids for water drip over Evap pad (rather than his drip tube)
Add float switch to fill top bucket then shut off until bottom level reaches float (saves battery power)
I used a larger fan inside a piece of 6" thinwall sewer pipe. Add sections & 90's to reach penthouse, etc.
I also added another row of holes in the bottom bucket between the horizontal lines since my fan was larger.
Playing with solar and an old battery to make it stand alone for tent use.
I also want my pump located in the top bucket (or outside of the bucket) so the bottom only has a tube.
I think this was a great idea but I have not yet tested in a dry area. His results show a 30 degree air temp drop and a 40 degree water temp drop (keep beer in the bucket)?? It should work really well in a dry climate...I will be testing during my summer travels.
The original design used for his rendition: http://www.instructables.com/id/Portabl ... mp-cooler/
Warning! If you look at instructables.com you will become addicted. Other Evap designs are also there but I think this was better than most.
Let me know what mods work for you (and if you have a 40+ gallon fuel tank for a 2003 SMB)