Time for a 3d printer I would think...

That is a tough one, because Max is very cool as is. My first though is sheet aluminum. Or if you search "flexible plastic sheet" on Amazon I could see that working well.

My first suggestion is to search SciFi movies and galleries to see what may meet Max's needs. I seem to remember a TV program which dubbed movies and the hosts were robots. One of them reminds me of Max's shape. Once you get a good idea, and draw it out, it would be easier to decide what to build his shell with.

I have seen on the internet DIY forums, a Vacuum Former. This would allow you a way to custom form sheets to fit Max.

Of course you could always do the foam and fiberglass method.

Richard R 's 3D printed idea is the coolest because you develop it on the computer, then slice it and print it.

Great ideas. I have a company that manufactures with sheet metal. There are several types to choose from. I also have also seen the diy Vancuum form on web and I think that may be an answer also.