In an adult-size potty chair if you can find one. The only thing I can think of that would work would be an antique one (or possibly a good reproduction) that one would place a chamber pot in. I would expect proper placement of the chamber pot to be a challenge, though.
Personally, I would rather clean a potty chair than take a shower, a bath, or both.
Also, if you have a chair like that you could practice there like someone who is just starting with toilet training and is still a bit confused about how the whole process works (if you wish). I havenāt tried this myself, but it might work.
You could also try putting on an adult diaper and squatting down in the bathtub or shower in your bathroom. That way, you are in the right room and that might help.