There are two ways to install a cage: doit properly or don't do it. Going half-way makes no sense as added safety is verylimited, the car is uncomfortable and the shell is not strenghened by the cage properly.

I used to think half-cages and things like that made sense but after some years with such cars around me I really think that makes no sense at all. You can go for a design with a bit lower door bars etc. but it's really worth to do basic layout as per FIA rules and have all the suspention pick-up points extensions etc. As i understand Rod only does top-seed runs with this car, if there is some major suspension/wheel failure the only kind of accident that will happen is masssssive roll and then you need really good cage/seat compo, preferably with HANS device really to come out of it like nothing happened.