Ways to block my toilet?

I would like to have a way to clog my toilet, in a way that is
-undoable
-repeatable, and most importantly
-fairly idiot-proof.

Any ideas? There are lots of things that seem like they would work but that seem to fail because of the geometry of my toilet opening (yes, that’s what he said).

I want to do this so that I can fill my toilet with a mild bleach solution to clean up under the rim where I can’t reach. (Yes, there are other better ways to clean under there. No, I’m not interested in them right now.)

My first thought would be something like a thick-walled plastic bag filled with gravel, or something like that? Not sure.