lapping a heavy heatsink

When lapping a large heavy heatsink like a Thermalright Ultra 120 how do you keep the heatsink from rocking back and forth? I'm trying very hard to keep the heatsink level but it wants to rock.

I thought about it but couldn't recall any posts about the problem. I kept thinking I'd have the same problem if I clamped down the heatsink and used a sanding block.

I just finished and I think I'm okay. I guess I got lucky. I used two different types of razor blades and a bright light to check for flatness before, during, and after lapping. The edges needed the most sanding. Just had to be careful and stop before I went too far. The cpu was a lot easier to lap.