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.
Some of my other works:
Soap, Sponge dish. https://makerworld.com/en/models/231790#profileId-303578
Ender 3 V3 SE/KE Side Spool Holder. https://makerworld.com/en/models/213760#profileId-301424
OralB-iO Toothbrush Holder https://makerworld.com/en/models/380407#profileId-280615
Saw / Router Height and Depth Gauge. Tenon Caliper https://makerworld.com/en/models/400110#profileId-301628
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