For limited opportunities to pee, I’d carry a good sized bottle and limit drinking to sipping water/an electrolyte beverage through a straw.
For limiting poop, I’d try to be empty ahead of time, and just take something solid like bread and cheese which means you probably could go through three days without pooping.
I need to survive for 3 days without pooping, and eating as little as possible. I can pee, but not very often. It can't take up too much space. What food do I pack?
Please don't ask why I need this....