Bumblebee Transformer Costume
Materials included a couple of foam camping bed rolls... and a bike helmet. Then we used a hotknife from a craft store to cut and sculpt the foam sheets. We used a low temp hot gluegun to put it all together, then coated the parts we wanted to look shiny with polyeurothane.
This type of foam accepted spraypaint well, it didn't eat it at all as it would with styrofoam.