This teeter-totter style mousetrap is open at both ends, making the mouse think it can just pass through. (You gotta think like a mouse.) But each entrance is actually a teeter-totter ramp that once it starts to tip, it gets pulled shut by magnets located in the tip of the ramp and at the top of the entrance hole. The two ramps on either side are coupled together by little arms (see last photo) so that both sides will close simultaneously.

The trap is constructed of two identical halves (consisting of the shell and the ramp) that snap together. To bait the trap, just smear a little peanut butter on the inside wall just above the plastic latch (see last photo). Then, with both ramps in the latched position, snap the two halves of the trap together. Place the trap on the ground and then push on the little nubs at the end of the ramps simultaneously to lower both ramps to the open position. Your trap is now set. To release the mouse after capture, just pry the two sides apart and let the little fella run free. Watch the video for a demonstration.

Printing
I printed this in PLA with 0.2mm layers with both parts in the orientations provided. Print 2 each of the shell and ramp. No supports are needed.

Assembly
You will need 4 Neodymium magnets 1/16" x 1/8" x 3/4" like these here. Use superglue (cyanoacrylate) to glue the magnets into the shallow slots in the ends of the ramps and in the shallow slots on the top edge of the shell entrance. Make sure you mark the magnets with a Sharpie before gluing in place to make sure the mating faces will attract rather than repel each other!

You will need to use short lengths of 1.75mm filament (4x, 20mm long) as hinge pins for the ramps. Position the ramp inside the shell and then insert a pin through the hole in the side of the shell and into the pivot hole in ramp. The hole in the ramp is tapered, so if you tap the end of the pin lightly, it will stay in place. Use a second pin to secure the other side of the ramp. (Do this for both halves of the trap.) Cut the pins off, leaving a few mm sticking out in case you need to disassemble the trap.