Plain, unglazed terra cotta pots and PVC pipe that doesn't have anything on it are just fine for tanks. I use both in mine. Use new PVC pipe so you can be sure it doesn't have any chemicals or solvents or anything on it.
As for making your own pots . . . I'm not really sure on that. I'm guessing any organic clay would work; make sure it doesn't have a lot of iron or heavy metals in it and DON'T glaze it before you fire it. I'd say no to things like Sculpey clay though. As far as I know, that's more of a polymer clay than a . . . clay clay.
Do be aware that depending on the type of clay you use and what its mineral content is that your water parameters could change depending on what leeches out into the water.