I work in the sign industry and use these products daily.
COROPLAST (corrugated plastic sheet).
Sign industry uses both 4mm and 10mm thick sheets. 4mm is very easy to cut using a utility knife. If you are going to be cutting a lot, a Coro Cutter that has guides for the flutes makes it go fast. For 10MM cuts we generally use a panel saw as it is pretty rigid. For structure substrate, the 4mm makes for fast construction using simple tools. End result is very light and may require added weight. Gluing can be tricky. I prefer a good quality double faced tape (3M VHB if you can afford it) over glue for Coroplast
Lots of folks use it - search the forums. I think Ken Brunt has posted threads on methods.
FOAMED PVC SHEET (Sintra, Azek & Komatex are just a few brand names)
Available in many thicknesses. Sign industry uses 3mm, 6mm, 12mm, 19mm and 25mm (Roughly 1/8", 1/4", 1/2", 3/4" and 1") for various applications. For structures 19mm makes a very robust substrate. Cutting anything thicker than 6mm requires a saw. Cuts and drills with wood tools. Ken’s recent Mancos Depot thread shows his methods. All but the thinnest can take nails or screws. If you use screws, don’t over torque as the threads will rip out. Lowes sells an Azek glue. Works great but it’s pricey. A generic PVC Pipe cement costs a lot less and will work.
For outdoor use, I think I’d probably suggest the foamed PVC board in 6mm or thicker as the superior product, It does cost more . If you have a local sign shop, you can beg for scraps.