So here's Nate with the finished product, two days early:
The sign on the side says, "Bank of Ireland - Extra Money Storage." And that's a ladder up the side because the door's in the top.
View of the door in the top and another sign, saying, "Top Secret, Keep Out, Money Inside."
The door opens to reveal a ladder leading down. The sign on the back wall says "Do Not Go Down Ladder - Money 1 Floor Below."
So then the leprechaun should want to go through the hole down another ladder to the next level, where he sees lots of money lying against the far wall. The plan is for the leprechaun to be so excited about getting the money that he neglects to look down at the floor. In so doing, he misses seeing the camouflaged hole in the floorboards.
And in case anyone is thinking why doesn't the leprechaun just go in through the front to get the money because it's all open, you will be glad to know that the top of the shoebox goes on when the trap is set; it's just off right now for illustration.