Check out www.bragdonent.com His system is a bit expensive, but it will make a 2'x4' module that weighs about 7# total with scenery. I saw his presentation at a GAT Show, and he had a tool box fall on one of his dioramas on the way down to So Cal from the San Francisco Bay area. The corner of the tool box rode on the module trying to dig a hole through it. The result, some damage to details on the scenery, no damage to the module base at all. I think it would be the ideal material to use to make a light weight, strong access bridge.