Not a cage but.....

I think you might end up torn between the two extremes of what amounts to a roofrack and a full roll cage.

If it is to protect against minor impacts you will have a choice of either deforming the "cage" or transferring the energy to the bodywork. If you decide to go for the non-deforming version then it could end up bending the bodywork that it attaches to , so you then move it down to the chassis and before you know it it becomes a roll cage.
Having protected the front pillar then the rear quarter will be exposed and suddnely you have a full exo cage.

Doesn't the Landrover version effectively bolt down to the chassis through the Wing tops?

An interesting challenge and not a simple one to work out. Keep any bar across the windscreen at the top and bottom out of the viewline as it will cause hassle with MOT inspectors. My full cage failed at one MOT station but passes at another. The MOT examiners text states something along the lines of "obstructions forward of the windscreen are not a failure" but some testers take the "sticker" rules and apply them to everything.

Yes, any full cage would need triangulation inside the car otherise it just collapses like any open ended carton would if pushed on the side.

Mine has a diagonal inside the car which bolts through the roof onto the external cage. However if I remember correctly it does not bolt through to the chassis below but goes to some spreader plates on the floor (it was fitted when I bought it). If it went to the chassis it would create an interesting problem where the car body becomes rigidly attached to the chassis rather than "floating" on the body rubbers. As it is my body is effectively attached through to the chassis by the roof bolts to the cage! There is still some movement between the body and the cage as the previous owner fitted small rubbers (door stops!) between the cage and the body and I can hear it move sometimes!

Martin