No idea where you could get plans, I built a leading link for the tiger rig I built several years ago. We have another tiger in the shop right now that is getting one of our M72D sidecars, On this one we are building "Leading legs"
The tiger I built we did up in orange with "tony the tiger" Stripes, It was GREAT! one of the most fun sidecar bikes I ever owned.
What we do to build a one off leading link (with a bike that is not wreaked) we put it on the bike lift and tie it there so that it can not move around. We mark where the axle is on the lift and figure out where we want the trail to end up and place a mark where we want the trail. We then build the link off of the bike to end up with the trail we want. We start by figuring out what size down tubes to use. We usualy try to use 1 1/2 inch tubing as this is what our adjustable building jig is set up for. By using a jig it allows us to make sure that the swing arm is square. If the fork tubes are larger then 1 1/2 inch we make sleave shims on the lathe so that the down tubes will fit in the stock triple tree. If the fork tubes are smaller but not by a lot we will turn down the tubes before they are bent. We use .120 wall DOM tube with a second DOM tube in side. So for 1 1/2 tubes we will have 1 1/4 tube down the middle. We then bend the tubes. Once we figure out how long the tubes need to be and have located where we want the bend to be we then notch the bottom of the tubes so that we can weld in a sleave on the bottom of both tubes that will hold a total of 4 bearings for the swing arm. We want the pivot point for the swing arm to be a bit lower then the axle. If we are running a car tire on the front we may make either the fork tubes end up wider or the swing arm or both. Many ways to do this. We also have to figure out how we want to mount the brakes. If they are firmly attached to the swing arm the front end will climb on braking. If we do not want the front end to climb we float the calipars with tie backs to the down tube. By adjusting the tie backs we can make the front end either climb, dive or stay level on braking. If you go to the Hack Venders section there is a build of an F800GS we did that has a link we built like this. Of course you also have to figure out what shocks to use, if you want long travel or not. How the spedometer drive is going to mounts and on and on. Leading links look simple but there is a lot to them which is why they cost what they do.