I sure like that Indian Flxicar. Judging from how it sits it is not a rigid attachment, but can flex on turns (guess that's why it 'flex' in the name. I see that your bike is also going to be able to lean into curves. Cool.
I made my sidecar body from a seventeen foot Grumman canoe that had been mangled in a rapids. I cut it down to bike length, removed the keel so that it could be narrowed up and then made a square stern end for it. I have also thought that a smallish Kayak would work. It also occurred to me that one of those roof carriers for cars that have a kind of kayak shape to them might be modified into a sidecar body.
Was just wondering about some sort of leaf spring at the attachment to the bike frame to retard the amount of flex on curves. Mine is a rigid mount and I've debated with myself over whether to use rigid or flexible. The old Watsonian sidecars for bicycles at the turn of the century were flexible, but most of the motorcycles that came a little later were rigid which I suspect has something to do with greater speed.
Anyway, I'm always glad to see a sidecar being built and it will be a looker combined with your Panther.
SB