On the rough tracks oz I found I needed something a but more substantial. After using a couple of boxes.
So I fixed a piece of black painted hardwood to the top of the rack so that the top of the wood was level with the rear lip, then screwed the mounting plate to that.
Worked for a short time but the mounting plate was not stiff enough as the catch at the rear ends up taking a major proportion of the load, and the flexing fixing plate started to crack.
Stiffening things up with 40x10mm flat bar backing a new plate helped a bit but then the latch at the rear becomes the weak spot.
The final answer was to screw down through the base of the box and the mounting plate into the wood block.
Used a couple of cable clamps over the loop at the back of the seat and into the box too, after you have lost a couple of boxes you tend to go for belt and braces.