I made a shopping bag cake (and it turned out really well....at first) I had support dowels in each layer and one that went through the middle of the entire cake and INTO the cake board. The cake fell over about 6 hours after I put the panels and everything on and finsihed it up. It seemed sturdy when I was done with it. Was there something else I should have done to make it sturdier?
How tall was it? You might have tried a thicker board. I often make my own cake boards of 3 layers of 1/2" foam board covered with contact paper. Then put two sharpened dowels into the cake all the way into the boards.
I was about 8 1/2" tall and I did a double cake board....so it was about 1/2 thick...
I would use THREE (or even 4) 1/2" thick foam board (the thicker ones - not the thin ones) - so the base would be 1 1/2" - not 1/2".
I use half inch foamcore under those types of tall, thin cakes.
I support every 4 inches of cake (on it's own board).
I use a small bubble level to make sure that each "tier" is completely level, and I always spike the cake to the board with 2 long dowels.