PVC doesn't just sit in the base cake or else it would be top heavy and plop about. It would actually be screwed into a wood or masonite base and the cake would come up around it. I cut my base cake (notched out) a large enough section to slide the cake around the pipe and then just popped the cut out pieces back into place, iced and covered in fondant.
I used rice krispie treats with an ice cream cone of top for the tower. I put a poly dowel thru the center of the tower extending out the top to attach the cone and a few inches out the bottom to attach into a hollow plastic dowel (filled with candy melts ).
A friend of mine made one and also used RKT to construct it. She got a thick dowel to place down the center. She actually traveled with it put together and it was very sturdy.
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