after last crash I found that foam will be too weak.
I'm still a noob pilot so I've broken quite a few props. To protect my props I've bolted on aluminium extensions to each arm that are slightly longer than the radius of the prop (see my attached pic). They're not as light as foam but they've saved the props on several occasions. My plan was to eventually attach a guard ring to these arms (either one per prop, or a large one around the whole quad) but this setup seems to be working fine.
I you have a quadcopter, you could make something like this: http://solsie.com/wp-content/uploads/2010/01/image8.png
You can probably make this by yourself by sculpting different materials like styrofoam. If you want something stronger, I would go with styrofoam covered by paper (or any other light fiber material) and resin.
I wish this helps you a bit!