I have a front fiberglass casting of a HIC and I want to adhere it to a hollow core door. Basically making a very thin version of the HIC. Can I use Bondo to adhere it or what other material could I use?
Polyester is a weak adhesive, even more so when fluffed with talc aka Bondo instead use a two part epoxy or polyurethane glue like Gorilla Glue... You could even use construction adhesive (liquid nails, organic tile/shower surround mastic) as it will provide a little 'give' between the different materials
I would not use gorilla glue as it foams (urethane and any presence of water reaction) and the foam becomes rock solid. I always think of it like using Great Stuff foam sealer to adhere things, yea it's sticky but it's very similar (urethane foam) and it just doesn't make sense to fight off the ugly foamy bead or displacement that comes with it.
I'm not familiar with the items you're gluing but for something with a lot of surface area, I always use 3M Super 90 spray adhesive, or even Super 77. If it's got a narrow wall or edge you're sticking down, 5 minute epoxy is great. I like to mix a batch, put it inside the item and up the wall a bit, then flip it over and set it down against the other item I want to glue it to. It will run down and seal against the edge inside, forming a bead with a nice clean meniscus all out of sight, then harden up in about 5 mins you're good to go. I even buy plastic syringes and load them up with the mixed batch, makes it easier to apply it.