This might work:
One rebuy or addon per person, no more. Stacks are 1000.
Worst case there's 150000 at play, putting the last level somewhere around 4000/8000.
15 minute levels.
300 minutes in total, minus a level to not break the deadline, minus just 2 breaks of 15 minutes, should leave us with 255 minutes of poker, or 17 levels.
5/10
10/20
15/30
25/50
40/80
60/120
80/160
15 minute break, color off T5 and T25
100/200
100/300
200/400
300/600
500/1000
800/1600
1200/2400
15 minute break, color off T100 and T500
2000/4000
3000/6000
5000/10000: should end here at the latest
10k/20k: if not, double the blinds to really end it!
If it still hasn't ended, split the payouts.
My 2 cents