I am making a birthday cake for my niece that will be 7 on Labor Day. I would like to make her a pillow cake, but her mother says her cake needs to be a 2 layer 9x13 (1/4 sheet). Apparently, she thinks there will be a lot of people at her party.

I have read the article on making a square pillow cake. However, since I want to make a rectangle shape cake, does anyone have an idea how far in from the top & bottom I need to carve, if the cake is 13" wide and 9 inches deep?

Alternatively, should I just make & stack 2 smaller pillows?

any ideas?