I know this isn't the easiest thing to do but have you ever been able to buy an empty box for a loose playset or vehicle you have and made it MIB? I've managed to do it 4 times. Bought an empty Hall of Justice box, an empty Mobile Bat-Lab box, an empty Batcycle box and believe it or not an empty Waltons' Farmhouse box.
I need to find a POTA Fortress box (#1 want) and a Green Arrowcar box and I'd be all set.
How about you all?