1st weigh in the day before, as it is.
2nd inside locker rooms 10 min before entering the octagon.
IF the fighter gained f.ex. more than 5% - cut his purse by half
there ya go
Major problem would be that top 10 fighters official purse is only a small percentage of their earnings per fight, but still - it would be revealed to the public how much they really weighed, causing a backlash from fans.