I have had pretty good luck with the Pelican cases I put on the front of my bike; I will be setting up something similar in the back fairly soon. A Pelican case weighs about as much as a similarly sized 1.5mm wall aluminum case. The cases are waterproof (truly), they can take a beating because the plastic flexes a little more than the aluminum, and they are lockable. They are also much cheaper than aluminum cases. My cases work as crash guards, and at $45 apiece, if I do detroy them, they are cheap to replace.
If you buy an existing sturdy rack, it is a simple manner to drill a couple of holes in the Pelican case for mounting hardware (just silicone the holes as you put it together to keep the case submersible). If you ever have a crash serious enough to destroy a Pelican case, and the rest of the bike survives, it will only take a hand drill to put the mounting hardware holes in a new case. On top of that, Pelican cases are available in many places. You can have them shipped to you or find them in larger cities, should you need to replace one on the road.