I already have a ton of rigid (XPS) foam, so I am going to start practicing making foam weapons and gear. For finer details, and to fill gaps, I was hoping to use a filler or sculptable medium that air dries and remains light; wouldn't want it to add weird cog. My niece uses Crayola model magic. I've also thought of using spray foam gap sealer, but that stuff really expands. As a last resort, I do have milliuput, but a lot of that can add noticeable weight.