I have just done this in my clio. What i did was to buy some heavy duty velcro about 2in thick, staple the fluffy side to the underside of the box, i used three strips. Then i trimmed round the box into the boot carpet so that the metal boot floor was exposed. i then used the adhesive side of the velcro and stuck this to the floor in the same place as the fluffy stuff on the box. It works really really well, and i have had no problems at all.
Chuckie