HACKS:

remember that that stuff has a lot of nasty stuff in it. you should definitely be wearing a full respirator, thick gloves and have adequate ventilation to prevent the buildup of fumes (and possibly a house fire if the water heater kicks on at the right moment!) remember, you only get one body - TAKE CARE OF IT!!

there's actually an easier and probably less toxic supply for making plastic parts. hobby stores carry a clear two part resin used for casting the clear plastic bricks (usually with a butterfly inside). it's about $5 for enough resin to make an actual brick sized chunk of plastic.

that being said, you can make a mold out of anything that wouldn't be dissolved by acetone. balsa wood would be a great choice, as would (cured)fiberglass or even plaster(really cheap at either the local hardware store or the hobby store). making plaster molds is extremely easy, and -apart from the dust during mixing- very non toxic!

heres a link to a manufacturers web site with a PDF manual for making plaster molds:
http://www.google.com/url?sa=t&ct=res&cd=1&url=http%3A%2F%2Fwww.opusframing.com%2Flibrary%2Fpdf%2F2part_mold.pdf&ei=xgHgRuy8CI_ogwOJr8ibAQ&usg=AFQjCNFwsWDvxxvMyfjdXo_dcxfB-uiNQw&sig2=6DGJku5grNa3S07KxqKWSw

there are plenty of other guides out there, just search for "plaster mold making"

I electrospin viscous polystyrene solutions as part of my graduate research, creating fibers on the nano scale, from 30nm to 10um. These fibers end up looking a lot like cobwebs, something most people try to get rid of.

Drop casting, spin casting, or molding are probably more tangible and useful fabrication methods. You could also use this stuff in a fab@home if you happen to have one of them lying around.

mrChew said
remember that that stuff has a lot of nasty stuff in it.
Are you sure you're not thinking of polyurethane foam instead of styrofoam?IIRC, polyurethane is formed by reacting isocyanates with water, and is used where you want to expand the foam in place. The downside is a lot of toxic materials, the upside is that it gives you one or two custom-fit solid blocks of foam instead of small bits that end up everywhere when you unpack.

That's an old modeler's trick. The sprue or parts "trees" can be dissolved in acetone or liquid model cement to make a filler that has the same properties as the model. So it's no more or less resistant to sanding, etc.

This "liquid" plastic could have a zillion uses as a rapid prototyping material, "oops" fixer, etc. Just make sure you're working in a well ventilated area! Oh, and after enough exposure to it, acetone and lacquer thinner cause my skin to break out...