a cinder-block is the most stable 'stool' i've ever come across. Others slip out from under the kiddo or can be off-balanced and the kid falls. Cinder-blocks may not be 'pretty' but they sure do work.
and I know you said you aren't doing a child seat, but this is a wonderful alternative. No seats to remove or put on. It's one seat with three parts - adult seat, child seat, and lid. I love ours. And their customer service is excellent. http://www.bemisseats.com/nextstep/
added: while my son sits there on the pot to poop, he uses a barstool to 1. prop his feet onto and 2. play with his Leap Pad. The rings on the stool are perfect height for his feet to rets on and the top of the stool is perfect height to play with his leap pad while he gets all the poops out. It's his special toy that stays in the bathroom at all times. The wooden barstool works great for feet and table-top purposes.