At my place of work we use polycarbonate fluted material for roofs, deeper than 5mm. It is strong and we often cut scrap of it up, it screams literally (3 x louder than timber) when cut (crosscut tee, jigsaw, table saw) . It is strong but as stated attaching things to it would involve adhesive(or building silicone/sealant) I think in time it will weaken and seperate, or appropriate screws (vibration may loosen these}. It is very light though, but once dented or damaged it stays like it and easier to do than say plywood. I just built first train tables (for uncle 4 joined up ones, largest ply panel 1200x600mm} used 12mm ply and batons. I would stick with the plywood (glue and tacking) although may get hold of an offcut and test the polycarbonate. Think it also has more flex than plywood.
