Here's one I scratched out using a 20 minute blind schedule. It has no increases above 50% until round 12, when it goes up 67%. With 20 players (I'm assuming 10 to a table -- I'd adjust this if its fewer per table), this should end within 1 round of round 14 (4:20-4:40 mark). You didn't tell us how many per table or how long the blinds would be, so I had to make assumptions on that.
Small Blind Big Blind Ante
1 $1,000 $2,000 $0
2 $1,500 $3,000 $0
3 $2,000 $4,000 $0
4 $3,000 $6,000 $0
5 $4,000 $8,000 $0
6 $6,000 $12,000 $0
7 $8,000 $16,000 $0
8 $10,000 $20,000 $0
9 $15,000 $30,000 $0
10 $20,000 $40,000 $0
11 $30,000 $60,000 $0
12 $50,000 $100,000 $0
13 $75,000 $150,000 $0
14 $100,000 $200,000 $0
After that, I'd raise the small blinds either 25,000 or 50,000.