The rear numberplate panel on the Citroen C6 is held to the boot lid by four screws along the lower edge. The bolts screw into threaded clips, but the plastic supports for these clips often break. These parts hold captive nuts to replace the threaded clips and their plastic supports.
Orient the parts for printing so that the screw holes are down, and the hex nut pockets are up. For example, refer to the image of the orange parts.
I printed in PETG, at a low resolution (0.3 mm layer thickness) and stuck the parts in place with 5 minute epoxy adhesive.
Is PETG the best material choice? I don't know. It should be tough and durable, but I'm not sure if it's ideal for use with epoxy adhesive. This repair does rely on the strength of the glue joint. Mine seem to have stuck well enough though. A nice low resolution print gives a bit of texture for the adhesive to grab.
There are four unique parts. Inner (smaller) and outer (larger) blocks. Each part is slightly tapered to fit, so they are in mirror image pairs. Make sure you have the right parts in the right places.
On my car, the original clip supports broke at the top, leaving most of the upright parts in place. Any remaining plastic helps to locate the blocks, but may need to be trimmed for a good fit.
Press an M6 nut into the hex hole in each part. You may need to pull it into place by gently tightening a screw from the other side, and/or hold the nut in place with a bit of glue. There's no access to the nuts once the repair parts are glued into the panel, so make sure they're secure.
The blocks slide in sideways as shown in the attached image. On the outer block, the protruding arms fit under an overhang, if that part of the original plastic is still intact. Note the recessed end face on each block. This is the narrow end of the taper and faces the outer edge of the panel.
A test fit is probably a good idea. Tape or temporarily glue into place, fit the panel, and see if the holes line up!
Rough up the plastic, and clean thoroughly, e.g with IPA, before gluing the blocks in place.
For panel removal tips see http://c6owners.org/plugins/forum/forum_viewtopic.php?11377.0.