I recently came across some plastic sign material. one variety can be seen at:
The material I have was about a 1/4" thick, and very durable. Difficult to bend or cut with scissors, and resistant to bending.
I wish that I had paid more attention to this material during Pirates of Penzance this past spring. Pirates needed pieces of material that were as thin as possible, but durable. I looked at foam core, but the price was prohibitive and I needed larger sheets than what I could purchase, and laminating them together was cost prohibited. i went with a luaun / foam sandwich, which I have used on other productions. On some pieces we inserted steel or wood supports (for a cantilever section, and for attachment points), but the foam / luaun sandwich bonds well, has nice edges, doesn't require internal framing except for special circumstances (these were all used as flats not platforms). They are relatively economical, and can be laminated in a variety of shapes and configurations, making them ideal for solid odd shapes that abound in theatre.
But, this sign material could have been advantageous. It would have been lighter, came in colors, can be ordered to be flame retarded, could be directly printed on, and comes in a variety of sheet sizes. The edges would not have been solid (the material is like corrugated cardboard except it is plastic) but the thinness of the sheet would have made that irrelevant.
Something to keep in mind for the future.