Getting closer to figuring out my battery requirements
You might try http://www.adafruit.com/products/353  Two of these in series will take a volume of ~ 140 mm x 54 mm x 18 mm, and will give you 7.4 volts at 6.6 A-hr. Assuming an 85% efficient converter, this will give you about 1.6 hours for a load of 5 volts and 5 amps. And, if you're a little flexible on your dimensions, you can fit 4 cells in ~ 140 mm x 110 mm x 18 mm, with a total endurance of ~ 3 hours. I don't know if weight is a problem.

With this form factor, theoretically you would then have room for a 140 mm x 100 mm pc board for your regulator, with about 10 mm thickness available for components. The realities of packaging, of course, will cut into this.