I've thought about creating a thread with all the inspirations I got when building mine because I spent a few months figuring out exactly what I wanted to do. I didn't want to lose a ton of space in the trunk, and I wanted a platform because there was a little kink where the seats went down that I could feel through my sleeping pad when I slept back there in colder climates while camping with the wife.
Give a look at my build and hope that gives you some idea. Ended up going with a drawer that I keep in permanently now (although I can remove it as its not bolted down), and then a detachable platform that makes it flush with the drawer. It's only about 8.5" tall so I do lose some space while sitting down but is good enough sitting on top with a slight hunch.
Link here:
http://www.toyota-4runner.org/5th-g...ystem-sleeping-extension-build-[5th-gen].html