I am curious why you are seeking alternatives to a wooden strongback?

If you are willing to spend $20 at the most you can get a really nice straight grained 2x4 at a lumber store (you can even get it in a stronger, more stable wood like oak if that is what you want).

Or, you can buy a nice sheet of birch (or other) plywood and make a box beam that is incredibly strong and stable.

Seems that the ease of use of wood along with the ease of fastening things to it with common tools would make it ideal.

Just my opinion - I'd be happy to hear why I should change it!