mine always poop in the large tupperware that i use to collect the water from the dripper.
I used to do that too, and my veiled would always poop in it.
Then I discovered dura trays, quite by accedent, on the web and now have one under each cage with a drain and collection system.
So I only need to empty a bucket.
I drilled several holes in the plastic floors of the cages for drainage, and stuck pieces of wood under each corner of the cage to give it a bit of lift above the dura tray.
Dura trays can be found from anyplace that sells rabbit cages and supplies.
Now, I never get little pools of collecting old water
And, to keep on topic, only my veiled has a designated pooping spot, and I know when he is going because he starts climbing down to the bottom on the same vine every time, gets in the same place, lifts his little tail, gives me a dirty look, and lets it plop on the floor
