First off, I'm not a pro, so there was no contract or date the cake needed to be finalized by. I'm just doing it for a friend's daughter's Sweet 16.
The cake was originally planned to be 3 round tiers. 6,9,12 with the SPS.
Now they've changed to square presents and possibly the middle being a slice of cake, if I can pull it off.
It's too late to order the SPS, plus I don't know that they have triangle plates.
How/Where would I dowel a slice of cake to support a square on top of it? How do I decide what size the cake slice will be? Do I transport it separately and assemble on site? How do I be sure it will all come together on site without stacking it first?
I don't use SPS so someone else will have to give you that advice, but my advice is to do the "slice of cake" for the top tier...much more stable than the slice in the center tier.
I liked it that way, too, and suggested it. But they want a present on top with a gumpaste bow as the topper.
I would try to get them to do this...presents on the bottom two cakes. On the base tier, put a tied bow on a top corner. On the middle cake, wrap a fondant ribbon around the sides of the cake and attach a tied bow on the ribbon, so the bow is on the side of the cake.
For the top "piece of cake" cut out a gumpaste 16 and curlie ribbons (which are about 1/4 " lengths of gumpaste wrapped around a dowel rod. After about 10 minutes you can slide them off the dowels to dry overnight.) Place the 16 on top of the cake and have the curlie ribbons shooting out from the center also. That way you still get a ribbon (present) look and you have more stability.
If you check my photos, you will see a birthday cake (blue and white)that I made for a football player that has the curlie ribbons on it.