We use expanded aluminum for the best of both worlds. We do not need to drill holes for the electrical belly pan, it is lightweight, won't shatter, and it is stiff enough to bolt to the bottom of our robots easily with 1/4"-20 fasteners and washers. You could also weld it I suppose.
Here is the page where we get it:
http://www.mcmaster.com/#expanded-metal/=118mlm3
We usually get the diamond shaped sheets. Cuts easily with sheet metal shears too.