Shape styrofoam to resemble a toilet with tank complete. Add a small piece of foil for the handle, and shape a piece specifically for the toilet seat so it sit on top.
I did this years ago when I built a house for a class. Had to furnish it so... imagination goes a long way.
Dental floss containers work well for the tank, J & J brand, classic type. If you use other methods to clean plaquey scum from between teeth, remember that floss makes handy string anyway.
Seems to me the bottom (rounded) end of a plastic Easter egg might make a good bowl. I would glue it to the plastic core from a roll of scotch tape for the base, then find a small box that would make a good proportional "tank" for the back. The seat I would probably cut from the plastic lid from a margarine container. Once the basic assembly was done, I would spray paint the entire thing the desired color, then add details (like the flush handle--for which I would probably use a piece of toothpick or wooden matchstick that I had painted silver).
If I wanted it to look really "realistic", I would probably get some polymer clay and make it from that.
Post photos when you're done, whatever you decide to do! It would be fun to see!
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