if you look underneath the car (to the left of where the backbox is/will be, near the passenger side wheel) you will be able to see a large metal plate bolted onto the underneath of the car (2 -3 bolts) this holds the rubber support for the backbox (where the support arm on the left of the backbox pictured above goes into) undo this and removed the whole thing from the car.
Cut out the rubber support (Renault one is crap and won't hold the Magnex system properly), go to somewhere like Halfords and buy a 'Rubber exhaust mount' IIRC its a Ford type, can't remember cost exactly (but its no more than £10) then use something suitable like epoxy resin etc, etc to secure it in the metal mount, I also used cable ties either side of the rubber mount.
Refit the whole lot back on the car, and put the exhaust into the new mount, I also used cables ties to secure the exhaust more at either side. When I initially had my magnex system it would never sit correctly, always hanging to one side, so I took it to a local garage and had the whole thing re-fitted, i think they removed a section of the system near the cat, they said this enabled them to re-position the backbox and get it to sit correctly (cost me £20).