I have dumbells up to 10 pounds only, then just bit the bullet and got a set of Power Blocks. I got the newest version, that adjusts in 2.5lb. increments, from 15lbs. to 50 lbs. You can also make them either 5 lbs., 7.5 lbs. or 10lbs., but unfortunately no 12 lb. increment.
Anyway, for the...