Sorry, I can’t find the specific paper-- NASA wrote so much about this (it was a big problem, plus astronauts had to save their poop-- which was then analyzed and copiously written about) that I just got lost looking. Search for “NASA low residue diet” and things like “preflight”, “fecal collection assembly”, “waste management system”, and etc – from Gemeni through Apollo.

There’s pictures online of the bags. They were taped to the butts to help make a seal.

Now I want to read the preflight diet study again, so please write if you find it.

NASA has a paper on how to not poop for days. It’s on the Internet. Before space toilets there was only a space bag with finger scissor/scoop holes. It didn’t work, poop got everywhere. The paper goes into detail about fecal matter being everywhere after early multi-day missions.

So they figured it out. Their system works – I’ve also had my own reasons.