IFI/VEXpro have a very nice battery cradle as part of their 2013 Drive in a Day Chassis Kit.
Item 217-2966 for US$25, shown at the bottom of
this page.
We just picked one up and will almost certainly use it for our 2014 robot.
It is formed out of 1/8" aluminum, so it is very sturdy, but not particularly heavy.
Cheers,
- Dean