When I made demountable sidecar mounts for Smudge's XBR I used the existing bolting points on headsteady and front engine mounts to fit a little subframe from which the chair was mounted. There was a clamp on the downtube, but this wasn't the sole clamping point and was therefore less stressed.
The rear mount was also a subframe that used existing bolting points.
Auxilary fittings were taken from other point on the lower frame rail.

I' d just like to point out that although I did the plans and cut all the metal I didn't do the welding. This was done by a colleague of Julian's and was superb work.
GC