Petfinder has the article and a very short video for the most basic of feral cat shelters you can make.
Two other things Id do that wasnt mentioned is: (from experience of making these types)
*Where you cut the hole into the container, melt the edges you cut for the hole, so it is smoother and don't make it too large a hole.
* Place it up on something if you have a choice. Dont set directly on the ground...