Designing a system is a matter of sorting out the parts you're going to need and finding suitable candidates. I always like to walk through hardware stores and home supply stores that stock all kind of gardening and other plastic containers and things that can be turned to hydroponics use.
He's a little float valve I found which I think might come in useful for my passive systems this Summer. I found this little value on ebay
so I bought a couple at $7 each. They are cute little devils, only about four inches and the seem well suited to controlling the nutrient depth in the subordinate growing tubs I envision for one of the variants of the passive systems. I looked at some other float valves too but they were bigger and made for animal feed troughs.