i got confused on the last ATM shirts as I saw a BM contrib and I thought thats what you were printing..
My vote is for a T shirt with a mock up pretend ATM.. Have a screen and keypad etc.. Card / reciept and money slot.. Brand it the 'bank of farang' of something and have ENTER PIN and 4 ****'s on the front (or incorect pin try again
)..
I think thats a classic one and everyone I have described it to thinks its a deffo idea..