Card box/bin/whatever

So, we want to have a place where guests can put a card or money where other guests won't take off with it. (We have to invite a couple people we dont trust. Okay more like I dont trust them. 1 is in the wedding party and cant do wrong but I've caught this person red handed. Another is the child of the aforementioned thief who is also a compulsive liar.) We dont want to make it obvious that we dont trust people, fh knows how I feel and told me to figure something out if I'm that worried about it. Being that we are asking exclusively for money (we have lived together for 7 years. We have everything we need/want, so money for honeymoon!) I need to make sure that nobody is being dishonest and hiring security for the card box is ludacris in my opinion. So, short of filling the bottom with concrete and making it a concrete box that's going to take a sledge to get into it, ideas???