I've heard of guys adding an amp behind the passenger dash pad. Just fab a bracket and mount the amp to it. Looks clean because it's hidden. Also, if you're building a low profile box for the back, you could always mount the amp inside the box.
I haven't found a good place for my amp since my speaker box fills up the entire rear cargo area (2 6x9's and 2 12"). Right now one of my amps is mounted to the box and the other just sits on top of the battery console.