As others have stated, the key to a good Spongebob costume is finding the right box. We got lucky – the new vacuum cleaner we purchased in early October was exactly the right size. I sprayed it with white primer and cut generous holes for the face and arms. Then, I spray painted eggcrate foam bright yellow and hot-glued it to the box. (This is a very tricky task. Take your time. Glue one side at a time. Allow extra foam if possible. And be careful because hot-glue burns are not fun.)
Next I cut slits in the foam for the arms, and a circle out for the face. (Save your scraps to fill in later if necessary.)
The eyes and mouth were cut out of cardstock and hot-glued on. (They were googly at the request of the costume wearer.)
Add yellow PJ pants and socks with electrical tape and we had one happy (contrary to the picture) trick-or-treater.