I am making a megaphone standing on end. I am wondering though. How do I put it together because of the slant cut. I need to put cake boards throughout so that it does not fall but do I just cut those with a really sharp knife? I don't want cardboard in the cakes. Any help would be appreciated.

trick -- premeasure cake boards for where they fall in the stack and then use them to guide the cut.

here's an example for a fairly large 3D megaphone cake.

hint -- draw it out on graph paper or using software so you can predetermine the sizes of layers and boards.

of course dowel as per usual w/ stacked tiers (which is really what this is)

Or if you are like me and I have good intentions.. but don't always follow through.

I would stack frozen cakes as Doug has shown. Do all of my carving and then un-assemble and cut cake boards to be the size I need.

Re-stack the cake w/filling, dowels, and boards as needed.