Bondo will not work the way you are hoping it to. Bondo is a filler, meant to build out cracks and dips in uneven surfaces (really for patching damage on a car). It has almost no structural strength unless it's like inch thick. At that thickness it will be REALLY heavy.
The honest truth is, it is cheaper to buy more resin, than to use enough bondo to be strong enough... and even then bondo will always be brittle, weigh ten times more, and the first time you drop it it will break.