It doesn't matter how big the base is just as long as the inside is the right size. Center the mousetrap in the middle.
Step 2: Sides
These are the sides of the bank. The left and right sides are the same size. The front and back are the same size.
Step 3: Top
The top has to fit around all of the sides. The inner dimensions are the same as the base. The slot is centered from the sides and is up against the inner side.
Step 4: Assembly
There is a certain way that this goes together. Set the mousetrap. The arm of the mousetrap has to hit the blocks on the sides to give it lift. In the picture the mousetrap isn't set but it is how it should look. After that is done place the lid on so that the slot is over the trigger.