exactly that. after 4 hrs of walking I put the gymnastic ball on the treadmill to sit. Getting used to it may take a while but I think its the optimal solution, as you will never be in a very static position. Regarding limited space, maybe you can bring the ball somewhere else while walking, I mean you switch once a day.
So what do you do after those 4 hours? You put the treadmill away and sit on the chair, you change the desks, you put a gymnastic ball on top on the treadmill to sit on it?
This question motivates me to rethink the whole operational strategy and to start using it again and maybe your answer would give a fresh idea how to solve my limited space issue. Thanks.