FWIW- Although would be heavy to use on it's own, it might be worth experimenting with epoxy thickened by partially curing it. I haven't tried it but System 3 describes a process in their instructions where you combine epoxy resin with less than it's normal ratio of hardener. As this cures it becomes much more viscous but does not set firm. When you want to use this thickened resin you mix the appropriate amount of additional hardener for a full cure and do whatever you are doing with it, it then cures to its normal state.
I don't have any idea how mixing in some lightweight filler might affect it except that if the resin gets too viscous it could be tough to mix the additional hardener into it.