I use 2 different brand portable fridges
the edgestar fp630
http://www.edgestar.com/products/fridgefreeze/fp630.asp
edgestar is the cheapest brand of these I could find. I got the 63l version for $320 as an open box
I also use the ARB
http://www.arbusa.com/Products/Fridges/20.aspx
It is considerably more expensive but much better built.
Both units work great and last a long time on battery power. I can't say for sure how long because I never really measured it.