V2 of my previous model, revisited because of the new contest. I cleaned up some porblematic areas which made the V1 version look a little bit unpolished. Also included a “pin” you can use instead of filament. You can find a video by Pete Firman below in which the magic trick is demonstrated.

After printing just connect the two halves together, you shouldn't need any glue. Also, leave the brim around the pin if you can, it helps holding it in the box. 

The way it works is that there is a coin already inside the box. As you push the slider (with the other coin) into the box you'll need to turn the box upside down so the coin falls into the first hole. Keep pushing the slider past the halfway point and flip the box upside down again so the coin, the one in the second hole, falls into the slot in the slider and you can pull it out through the opposite side you put it in initially.

You could print in different colours obviously. I don't advise using two colours for the box, since it will be easier to see the “magic” as you turn the box upside down to let the coin fall into the hole . I printed the model twice.