You can not
You can use the pans you have in your home. Uses a small 8x8 pan or even a 9x13 plus you will need a cookie sheet that has edges. You will have to layer if you use the cookie sheet. But if you use a 9x13 you will need to cut layers and then do edges. Its really pretty simple.
Maybe...You could just shape it with your knife or make a book shape with cardboard and put it round the edges of your cake? But don't put the cardboard in the oven! Just hope it stays up when you shape it...lol...I have the same problem...I don't have a cake tin!