when me and a mate made one for the challenge truck we made a rear hoop with cross brace that mounted to the top of the chassis reinforced with 8mm plate and the rear stays went to the back of the chassis just before the rear cross member mounted on top of the chassis reinforced with 8mm plate.
you really want to mount it to the chassis not rock sliders, and the hoops want to be made from one bit of tube not several bits joined.