Use gorilla tape!! What i did was, do a layer of tape then cut come cardboard to fit in the big areas then do another layer of tape, it really helps make it stronger
make sure the box doesnt move at all or it will rip the tape off.
there also might be a bunch of holes in your rear deck... look from inside the trunk and if there is holes... tape them up too
I prefer gorilla tape because its reallyfucking sticky (has ripped skin off fingers..) and its thicker and sticks to anything and everything