Seems like they're just molded into the panel. Your best bet will probably be to take those panels off to make sure, and look for an appropriate fastener. Do a google image search of "standoffs" to get an idea. If aesthetics aren't critical for you, you could probably make do with some type of eyebolt. Keep in mind that with any of these fastener fixes, the plastic panel is thin so I'd use a good-sized washer on either side of the panel to distribute the strain over a larger area. So it would be fastener-nut-washer-panel-washer-nut. Keep eye safety in mind. Try to avoid anything that would present a hazard to someone poking their head around back there.
If you really want to avoid removing and drilling the panels, one of those adhesive-backed hooks might do the trick, but I'm not sure how they'd hold up in the heat of a parked car. I did use some 3M "industrial velcro" to hold up our toll transponder and it's done well.
Or, you might be able to get those panels cheap from salvage.
I noticed you're still working with polymers.