The hitch is not really that hard to build, as it consists of 5 pices: 2 longer side arms that run from the front bolt hole to the end of the hitch, 2 drop arms that drop down from the turn signal mount and connect with the side arms, and a cross-brace that provides the hitch and safety chain points. However, 2 criteria must be met.
How high off the ground shoud the cross-brace be? My thought was to put it in line with the swing-arm pivot points. This would make the center of the pulling axis in-line with the pivot points, minimizing the effect of the trailer on handling. To get this height I had 2 helpers assist me. First, I dressed and loaded the bike as I would for a normal ride, then got on the bike and off the stands. 1 helper was assigned to hold the bike upright while the other measured the distance from the ground to the center of the swing arm pivots.
How far behind the rear wheel should the hitch be? You don't want the hitch to interfere with tire changing and maintenannce, but a less obvious concern exists. Trailer hitches can fail; that is why safety chains are required. If the hitch fails the tailer tongue will drop below the hitch and move forward while you are getting on the brakes. You don't want the trailer tongue to hit the rear tire!
My chains let the tongue move forward about 5 inches, so I needed to have the hitch at least that far behind the rear tire. I gave it another inch to be on the safe side. With a side arm loosely bolted to the front mount, and a drop arm loosly in place and clamped to the side arm with a C-clamp, my helper raised the trailing end of a side arm off the ground until he had located a point 6 inches behind the rear tire and the same height off the ground as the swing arm pivots. We tightened the C-clamp; this gave me the correct length of the drop arms, and marked the side arms for the correct length. A few cuts and welds later and the side arms were done.