Cheap Styrofoam
If there are any large construction jobs going up nearby, Hotels, strip malls etc. thet usually utilize the 2 inch styrofoam as a skin to th building followed up bt a few layers of stucco. After the stucco is applied, they spray the stucco and viola instant wall. I know that all the jobs sites that i have been on, many boards of this stuff have been discarded as well as decorative Moldings ( made from styrofoam as well).I know they wil be happy to give you the stuff, no sense filling up a dumpster with lightweight styrofoam when dumpster rentals usually go for about $500 A POP. Just a thought. Also, when the light fixtures are delivered, many smaller styrofoam (beaded) blocks aretossed as well. They can be incorpoarted into your scenery. I know if you went to an arts and crafts shop, styrofoam is not cheap by any means.