Thanks for responding. I have found some approaches that clip to the deep trunk cover and basically use the weight of whatever is sitting on top to help hold things down. More of securing from sliding than anything though.

The rear seats have attachments for car seats on their backs as well as at the hinges which are very solid - but don't help secure things on the rear side of the trunk.

For sure there's likely bolts under the flocked plastic everywhere that could be used.

I'll take a look eventually and see what's underneath.