I could think of even more offensive items to put in the contribution boxes, especially at the a$$emblies. Putting slips of paper with Satan's sigil on it would make them go berzonkers. Someone could even draw a black swastika in a white circle and a red background, and put that slip in the box (try that at a Grand Boasting Session, and they will wonder who is doing it). You could also write "Christianity is Communism" on a slip of paper and put that in the box. Or that Jesus is nothing more than the model of the perfect slave, and following its footsteps makes you the perfect slave. Imagine getting 15 or 20 of those at the Grand Boasting Session during the final session, when they have their fake deficit.
Another good trick is to go find some of those Zero Dollar Bills and Void Checks and print a bunch of those out. Put those in the boxes, preferably during the last afternoon when they are trying to make up for their fake deficit. You might try this if you are not up for the swastikas and Satanic messages. Blank slips of paper will also work, along with slips of paper with printed or written messages of how the washtowel is corrupt and doesn't even abide by its own LIE-ble.
You could also take some toilet paper (the stuff on the roll, if your currency hasn't already become worthless) and stuff that in the box. If your currency is such that it takes a sextillion of them to buy a single sheet of toilet paper on the roll, you could stuff the boxes with singles. Since it will take more than a sextillion of them to be worth anything, you stuff a few hundred singles (preferably crinkled, so they fill the box) into the box, and it will not be worth counting them all. If your currency can still be reasonably exchanged for merchandise, use the stuff on the roll (paper towels will also work). Putting in rubbish will also work--nothing like having to fish out all the cereal box tops, McDonald's bag fragments, and whatever small papers that normally would be rubbish in order to count the bills that really have value. If you have access to Zimbabwe dollars (the type that is worthless) in single dollar denominations, not fit for collectors, use those.
Whatever you do, do NOT give them anything of value. If your currency still has reasonable value, do not put it in the box. Do NOT put in a copper penny. If in doubt, tap it lightly with another metal object--if it has a clean ring, even once, it is real copper and should not be dropped into the box. In the United Tyranny of Stupidity, anything minted before 1982, and some 1982 pennies, were copper. In places where pennies are defunct, you should never drop them in the box as they have collector value. Nickels, with their metal content being worth more than face value, should also never be dropped in. Under NO circumstances should anyone ever drop in silver and/or gold. And I do mean NEVER! Drop in silver or gold, you are paying for your own damnation in silver and/or gold.