This made me think of an idea @Paul_Danyliuk had posted dealing with attendance in class and how to keep people from cheating. You could create a list of codes, maybe 6 digit numbers, then pass out slips of paper to each attendee, if the code isn’t contained in that list, the vote doesn’t count. If you print them up like business cards, you could have a particular company sponsor, and pay, for the printing as well as include other info, ad or otherwise.