Armor needs to be strong, light weight metal does not mean strong. If you need really light weight, and want real weight for strength while keeping it as small as possible use corrugated plastic board. The stuff is JUST like the normal cardboard you've seen, but made out of plastic. Place alternating layers 90 degrees apart from each other and make it as thick as you want. For it's weight it will last longer than anything else, against anything except a puncture style attack, and even that would have to have some decent force behind it to penetrate it if it's using cross linked plastics.
The vague clue I got from the picture was that it was twice the size of a human hand and light enough to be held up in it without stress.