Aluminum is another great material. Thanks to the automotive world, there is now some really excellent aluminum adhesive. Between that and rivets it's easy, and light to work with. No worries about water intrusion either.
I built a 9" raised platform that was strong enough for me and the dog to sleep on out of 10ga (1/8" thick) aluminum plate. Metal scrap yards are a good place to look for aluminum. Worked great for us, because I could stuff the whole bottom side with camp gear, and keep the top side open for Super-Mutt v2.0 to move around.
There is also different aluminum building block sets, that are pretty cool. The only downside is they are cubes, so it limits the flexibility to use every square inch of space.
Also, a good rack up top for lighter stuff can really free up the interior. We use a Yakima Rocket Box, which is about 20lbs, and can be stuffed with all the bulky stuff like sleeping bags, pillows, tents, mother-in-law, clothes, towels, etc..