There is in fact a shield test. It can't "taco", which means it can fold in on itself. My personal opinion, don't make a punch shield THAT big without a core. You could easily get a tupperware lid, or a tobbogan and use that for a core instead, and it'd still be light enough for a punch shield.
Wouldn't do you much good, if you ask me, considering heavy weapons would blow through it like a knife through butter.