This traps a mouse inside and locks them in. 

Assembly
It's pretty straight forward. First, put the lever platform inside of the main box. The thick end should be towards the opening. You will have to bend the side walls of the box slightly in order to put the lever in. Next, put the two skinny arms into place, there are two slots where they fit into the lever snugly. Then put the top cover on, bending the outer hinges of the box slightly to fit them into place. Then the trap door simply slides into place of the top cover once it's closed. You may want to sand the sliding door after checking how it fits. When closing the top cover, you may have to bend the two arms back slightly while fitting them through the two slots in the top cover.

Setting the Trap
The top of the trap opens, and you can place peanut butter or some other type of bait on the shelf in the back. Lift up the trap door so it's sitting on the end of the two arms. Once a mouse walks in close to the bait, the lever platform will move the arms, allowing the trap door to fall into place. And once the mouse moves to the door to try and open it, the arms will hopefully lock the door from the top. 

I have caught three mice so far, one with a transparent PETG trap, using a piece of a Clif bar as bait, and two other ones using peanut butter as bait.

Let me know if you find failure points are in this trap. I did have the mouse chew through one lever arm, but it still couldn't get out.