this shows an old, broken undertray alongside a new Peugeot one
This is the new fixing sold on ebay. I couldn't get them to fit tightly in the square hole and they often dropped out if I didn't line the undertray up accurately. The original Peugeot fixing was much superior, though tended to rust out.
so Plan B was to use these M6 Á M8 threaded rivet nut inserts (aka rivnuts)
I had to use mostly M8 ones which is rather overkill, but they fitted the holes well!
steel ones were very tough to fit, but then I got some aluminium ones which squished well into the square holes.
The new undertray was initially secured by these three original Peugeot clips at the front . . . . . . .
The new undertray was initially secured by the three original Peugeot clips at the front and eight M8 socket-cap bolts.
These clips have been replaced by M8 button-head bolts . . . .
The new undertray was initially secured by M8 Allen bolts (1) plus springs and flat washers.
The button-head screws (2) looked tidier but also needed washers.
Finally, M8 and M6 flanged socket button head bolts (3) not needing washers made a neater finish