A simple cage trap is probably one of the easiest traps to make and many people started trap making with a traditional home made rabbit trap. The same principles can be used for making smaller cages for rats and squirrels but those clever predators usually need a lock or spring on the door.
The corners need to be neat, straight and square so that the door and treadle can move freely.
The door can be held open using a straight rod attached to a treadle on the floor of the trap. A rabbit walking to reach the bait will tread on the treadle, pull the rod down and release the door.
In these photos a Larsen Trap spring has been added to the door to make it close more quickly and remain closed.