You don't have to make it a criminal offence, when they send you your voting card there could be a serrated bit you rip of and get stamped, with it you get a bit of a rebate on your council tax, without it you pay a bit more, we could call it the
democracy rebate, they could also introduce freedom cakes and democracy hot dogs like Australia. You don't have to make voting mandatory in a democracy but you can incentive it because for me without it you get extremists and populists with their simple answers to complicated questions and the antidote to that is mass turnout.