This is based on a set of these I saw in the trunk of a new VW golf. You make 4 of them so you can put them at the corners of a box, or a shopping bag to stop them sliding around when driving. You need to print 4 and stick the "prickly" side of Velcro to the bottom. The carpet or lining in your trunk should act as the softer part for the Velcro to attach to. They can easily be moved to accommodate different sizes. Also you can make bigger versions if you like. We tested a 3" version today in a Mini Clubman and the groceries stayed in place.
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/5x8cGIBhd3N

I used the default settings for my Replicator via the MakerBot software., so feel free to use your favourite settings.
You need to print 4 , one for each corner. You can scale up as you require either in Makerware or edit the file on Tinkercad.
Attach Velcro, the prickly kind to the the two arms of the base. The lining of the trunk should act as the soft part so the holders can grip.