The box is basically a tunnel about 30" long, with an inside width of 4" and a sloping lid.   One end is open, the other end is sealed but has to let light in, so it can be covered in mesh or drilled Perspex.

Inside the trap there is a seesaw of thin plywood positioned so that when the trap is set, the end seesaw is touching the floor at the tunnel entrance. This provides a gentle ramp but there is still daylight visible down the tunnel. Squirrels and rats seem determined to investigate any sort of tunnel and go in without bait. I've had a lot of success putting these in places between cover so they dive into my 'tunnel' as they're scampering across. They also work well positioned on planks or other natural ‘bridges’ across rivers or ditches.

Once the rat/squirrel goes past the pivot point, the seesaw tips and the entry 'ramp' rises, touching the roof and closing the box.  The key to this working is that the seesaw is long, so that once the rat is over the pivot it's already a well into the tunnel. If you've got the balance right then the seesaw tips pretty quick bringing the front of the seesaw up to the roof - closing the trap. You can mess about getting the balance just right by gluing bits of lead to the seesaw.
The clever bit is a wire 'U' that hangs down from the bottom of the seesaw and prevents it pivoting back.