It's not the most polite of topics for dinner discussion, but the simple fact remains that there are segments of the population who use a commode. It's feasible to make a simple one from an IKEA chair and a few spare parts.
IKEA Hackers has the details. It comprises a $29.99 Stefan Chair and a few extra bits of wood. In the case of this particular hack, the author notes that they scored the extra seat pieces from the as-is section in Ikea, which could also simplify things a touch. Even if you're not in the target market, it's an interesting application of materials, if nothing else.
Now you see it, now you don't potty chair [IKEA hackers]