You can't use bondo on a hot glue reinforced helmet. When it flexes from the hot glue, the bondo you applied will crack because it is rigid but brittle. As far as I know, there's no sandable solution for the exterior to smoothen hot glue armor, nor can you sand down the high ridges through the paper into the hot glue. It just gets gummy and wrinkly. If you're going to continue with the hot glue, I'd return the bondo if you haven't already cracked it open.
HOWEVER, with a good paint job on the HD peps, you can get a good-looking set of pure hotglue armor (you'll still need to pad the interior and whatnot). Just check out Pooper's hot glue armor as an example. I think he's the only one with a completed hot glue suit.
Your other option (I'm just thinking outside the box here) is to use another fiber like burlap or cheesecloth since your parents are against fiberglass, but you'll still need to encase it in resin. The tried-and-true process of resin outside, resin inside, fiberglass inside, bondo outside would remain the same. You'll just be replacing fiberglass with some other fiber. It just won't be as strong. If it was the resin fumes that your parents were actually against when they refer to fiberglass, then tell them that resin and bondo are literally the same product, but bondo has talc as an additive. You'll still need all the same safety gear: gloves and respirator.
If your parents weren't okay with the toxic fumes (understandably, depending on your age), then a non-toxic alternative is aqua resin, which is a water-based resin. You can still use fiberglass with it in the same way as polyester resin, just more expensive but non-toxic.
Just do a search for all the above alternatives I mentioned if you need more info. It's all here on the 405th, which is how I learned about all this stuff.