Milliputt is probably a little harder than styrene, but is still sands, drills, cuts, files, and grinds easily.
If it is a big piece you may want to use an amature, I would also use plumber putty, another two part epoxy putty found in the hardware store, to get a real rough shape made and finish with milliputt. Milliputt is expesive and there is no sense on using a ton of it just to take up space. Use the cheap stuff, plumbers putty for that and do you final work with milliputt over your rough structure.