I just use plain 22mm or 19mm shutterply, or go hunt up an old kitchen counter top. You can use doors ( they are under $20 new for hollow core ones) and they work well. I made a few worktops using melamine counter tops with a bull nose on one side, and as they are 50mm thick they can be quite sturdy. They are hard to cut as my skil saw only goes 48mm deep, but a 2 pass cut is fine in most cases before assembly, and leaves a smooth edge as well.