Update: I now include a revised version of the design. You can pick one you like.

This small hook sits at the edge of the retractable trunk cover. When you lift the floor to access the spare tire compartment you can use it to to keep the cover “open” by attaching the floor strap to it.

I've printed this on a side with no support. One hook side that's printed in the air wasn't perfect but trimming it with flush-edge cutters helped a lot. Support would solve this altogether. 20% infill and 2 walls is plenty strong.