How do the Purple Boxes work?

I've seen the purple boxes in a few maps, and I know that it stores what item you have in your hand. But I have a couple questions:

• How long does it store the item?

• If another player puts a different item in, does it change it for you? Or will it give you the same item you put in?

• Does the box save across matches?

Me, my brother, and a couple of my friends were wondering about it.