Alistair showed me this simple jig. It lets you position the rack top easily. I wish I had some photographs of my earlier "jigs", this is much much simpler and more effective.

The key part to it is a double holder that came from a local science store (called Science Art and More). I don't know the exact name of this part. It has two tube clamps and a center clamp that you can rotate 360 degrees. Attached to this are two homemade tube holders, one of which clamps to the fork or frame and the other clamps to the rack.