On tool monger, they posted a blog about Attic Dek; an open panel made for spanning joists to create a storage location or walkway. They claim they are lightweight, and are gray, with a thickness that suggests recycled materials. They support up to 250 pounds and come in 2 sizes- 16"x16" and 16" x 24".
The panels could be a reasonable choice for shop built storage or scenic pieces. At a price around $2 a square foot plus the ease of installation means that the total cost is very reasonable.