A few years ago, a mold-making friend turned me on to Piranha Putty instead of Bondo. Bondo is well-known for voids and how it doesn't work well when you need to fill them. Piranha doesn't make the voids, and it sands much better than Bondo. I haven't tried comparing it weight-wise for use as a part of an airframe, but when it comes to making a quick mold it's the cat's meow.

Andy