Use self adhesive velcro at the bottom.
It can be adjusted to various shapes. It will hold a box in the corner, a bottle, a container of windshield washer fluid or a shopping bag.
Thanks to the hinge, the possibilities of adaptation are endless. Two diagonally placed, are enough to secure any item, even in the middle of the trunk.
Use small layer height.
The author marked this model as their own original creation.