First build a regular MonoBand J-Pole from the Copper Cactus plans for a 2-meter antenna, leaving off the top pipe cap.
Build a second J-Pole, which will look like an L-Pole, the vertical is 38-3/4 inches from the top of the antenna, down to the centerline of the horizontal member.
The lower leg of the L consists of two horizontal pipes spaced as close together as you can get them using standard 90 degree elbows.
From the centerline of the vertical, to the centerline of the fittings at the end of the horizontal members the distance is 18 inches for 2-meters.
I have not calculated the exact measurements for any antenna other than the 2-meter band. A final 90 degree elbow is placed directly under the vertical section and is soldered to the top of the existing J-Pole.
The finished product resembles an L on top of a J, of course the lower leg of the L is two pieces of horizontally positioned pipe.