I havent done it but I was going to tackle this project for my sons 30th birthday so I've been looking at them. I would do about 4 or 5 layers of frozen pound cake and carve the toilet from that. Once you get the outside carved you can carve a hole in the top for the bowl and use piping gel as toilet water. The lid I am going to make from Rice Krispie treats put on dowl rods that can be inserted down into the cake so that the lid will stand up.
You could always make a smaller version of it out of krispie treats and dowl it down onto a sheet cake. I would think either way would work fine.
Good luck!
Im sorry, I didnt really answer your question. I would start with either 9x13 sheets or maybe 12 inch rounds or squares. That would give you more cake to work with.
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