These are inserts for the trunk of the Audi S5 (and others) that allow grocery bag hooks to be mounted. I guess they figured sport coupes didn't need them, but I think they need them the most. So you can still take quick turns and fast acceleration!
The hooks themselves can be had inexpensively via Amazon
I printed these with standing up with support in PLA (all I had at the time). Flexibility of ABS would be a better choice. Might be able to print this upside down without support.