With a decent ticketing system pay on the gate could be as simple as contactless - set it to allocated seats on the receipt - no choice on the day - and charge an extra fee for being too lazy to pre pay. But have terminals in some gates for this. Obs would need to be set up but surely its possible - maybe even self service ticket machines outside the ground (I'd make them contactless only to save the problems of cash. Future work but in this day and age should be doable.