We build a lot of sidecar rigs around the Bonneville, Doing two of the Steve Mcqueen editions at this time. For bikes that proper bike specific mounts are made for such as the Bonneville adding a sidecar is not all that hard, We ship out many sidecars in crates that people in stall at home. Our most popular sidecar for the Bonneville is our M72D. At this link you can see a photo on the Bonneville http://www.dmcsidecars.com/sidecars/m72d/
When ever you add a sidecar to a bike you end up with heavy steering. A leading link is one way to reduce trail on the front for lighter steering and when forks use to be a lot smaller diameter this was the only way to go. Now that fork tubes are getting larger another good way to go is with triple tree's that change the angle of the forks. We make new from billet tree's for the Bonneville. The one in the photo on our site has had this done. The tree's keep a stock look to the bike and still lower the steering effort.
Feel free to contact me with any questions you may have.