Here is a picture of one of our subframes for a 1200 GS. We build them to go down both sidecas of the bike and connect in the front and also have a cross member under the seat area. Use as many pick up points as possibel for attachment.We use the center stand bolts the hole across the front of the engine at the bottom, the seat subframe area and also sometimes the front crash bars for added support. You can attach in other areas but be careful as some bolts do have oil behind them :-)
The subframe below is ' in the rough'. We have since changed the style some. This particular one has been to alsaka across the US a couple of times and is heading to africa soon with no issues.
Note that the 1100GS and even the 1150 GS Bikes even in solo trim were known to crack the bike frame just ahead of the rear pannier down low where two members come together even as a solo. For a while some (not us) did do a simple clamp on bracket to hold the upper rear mount in this area and we have replaced many of these types of assemblies to correct a bad design.This is one reason we feel the need for a stout cross member below the seat connected to both sides of the bike. Another reason for a stout subframe is to allow it to take the hits that a skidplate may call upon it to do. More to this than just hanging a sidecar onto the side of a bike. Lots of forces from many angles are sent into any mounting system.
As a side note we do not care for the use of heim ends (SPHERICAL ROD ENDS) when mounting a sidecar. They do make assembly a little easier but they also do not provide any resistance to movement on all planes. They are, by design, made to rotate. This translates into more responsibility required by the upper struts and their attachment points. We feel that all mounts should share the load as equally as possible or at least provide as mush resistance to movement as possible.
Yes, some use heim ends and do okay but we just would rather add a little more resitance to the hinge factor that can is present with the lower mounts. After all Heim ends are made to swivel. If the lower mounts are an eyenut to eyenut or an eyenut to clevis connection there is a built in resistance to movement at the connection. For more resistance they can usually be turned 90- degrees after the setup is made. More invloved to build? Yes. Overkill? Maybe but so what!