I did a toilet bowl cake a few weeks back, same concept in that it had an open lid. I did the lid out of RKT with a cardboard backing. I put skewers up into the lid between the RKT and cardboard, and then just attached it that way to the toilet seat. Worked fine...I just didn't transport it with the lid on, waited until it was at the final location. You might need dowels instead of skewers depending on how big the cake is/how heavy the lid turns out...if mine were any heavier I would have needed skewers (my lid was formed from an 8" round).
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