West systems have a filler product called "Microlight" I think this is essentially polystyrene micro-balloons .. the resulting cures resin can be extremely lightweight and I have successfully mixed it with West Systems "High density filler"
That way you can create different levels of paste consistency and also hardness of the finished product. Hardness and "sandability" follows the content of filler and filler-to-resin ratio.
Using only "microlight" filler and mixing in the maximum amount of filler, the resulting product is almost as light as styrofoam, but quite a bit stronger, and sands nicely, but gets a porous surface.
Using only the max amount of "High Density Filler" the result is as heavy as pure Resin, but extremely strong, and will hold large bolts and fasteners in wood and composite materials. You can actually cut threads in it and screw in bolts..
The West systems fillers are not cheap unfortunately .... maybe Easy Comp have something similar, that might be cheaper....