Me personally.. I would buy a small pipe bender and fab the entire thing complete with a windscreen. A shopping cart is gonna look like a shopping cart no matter what you do. You'll also have to factor in the amount of suspension travel a dualsport has. The little Asian side cars are bolted directly to the frame (step thru floor section) and tail section of a small scooter that has very little suspension travel. On your bike it would cause you to pitch side to side as your tail section travels up and down (which TBRs are known to be bouncy anyway).
I would connect it to the lower portion of both frame rails just forward of your shifter with slightly larger pipe that the sidecar frame could slide thru, and the end of your right side swingarm. Both points would pivot and the front mount would provide the needed lateral stability. and if both mounts were permanently mounted it would be relatively easy to put on the sidecar when needed. Also the rear mount would need to be slotted front to back about an inch to allow for swingarm arc.
You would still experience some pitch as the suspension travels, but much less than if it was attached to the tail section. As far as sidecar suspension you can buy some really small leaf springs for little trailers. And go with a memory foam wheelchair pad as well for his added comfort. Just my 2 cents..
