I have a "U" shaped pan that I used when I did one. I put half set blue jello in the center along with gananche peices of...I am sure you can figure out what it was. I also made lemon jello and dribbled it on the seat and put some on the blue jello too. I made toilet paper out of fondant and smeared ganache at one end that was wadded up, it was draped on the toilet seat falling into the water. I wrote happy birthday in ganache and put a small pile of ..... next to the recipients name. I used a 9x13 cake to make the tank. My soon to be BIL loved it and now wants a urnal cake for next year. I hope this helps get your creative juices going.
Thank you so much for your help Lorendabug! I do not have the U shaped pan but WOW your cake is great! It's for my brother's 40th this weekend. I was told cake was taken care of (my SIL is letting her sisters do his cake when they have never done anything) but gag gifts are welcome! So what if my gag gift is edible (and is the best tasting cake there)!? I wanna try one! 
Thank you!
The toilet cake I made was my soon to be BIL birthday present, he loved it. I made another cake for everyone else. I just could not eat the toilet cake it was to nasty looking. Even when it was cut it looked nasty, with the dark fudge cake and the ganache, ewwww. I am sure your brother would love it. You know you could just do a round cake and cut a bowl into the top layer to put the water in, etc. Have fun with it!
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