Car appreciation... the aesthetics, the engineering, etc

Does your boot have hooks or tie downs on the sides?

The Superb came with hammock-esque nets that clip on to the side of the boot and are really good at holding items that might otherwise slide or roll around the boot. Probably not big enough to hold more than a weeks shopping across the two of them, but depends on how much you buy at a time.

Other option if you have those clips on the side is a bungee cord that separates the boot into two halves (front and back or left and right would both work). Ensuring the shopping is in some bigger bags that can't cross the bungee "threshold" would work.
Just seen that Dammit suggested a similar thing that but using a more rigid solution.

Last option that's common is if you have tie downs on the floor of the boot, a big luggage net that covers all or most of the boot floor. Your shopping goes under the net which is made of stretchy cord so it's trapped in place and can't move about unless you're doing some "spirited" driving.

Not endorsing this particular site, but they sell a variety of luggage net solutions:

https://www.caraccessoriesplus.co.uk/categories/all/all/luggage-nets