After taking out the driving lights in the front bumper last weekend and managing to shear 4 of the 6 M4 screws, I left them soaking in GT86 (WD40 but smells nicer

) for the week and had another shot at them today. No joy.
After some playing we realised that they were rivet nuts and would push out backwards - soon all 4 were out. Only issue is that they aren't a type I've seen before, actually having an M4 nut profile that is captive in the bumper moulding to stop rotation:

All the rivnuts I can find online are the round type with splines to stop rotation.
Anyone know what it's called or where I can get new ones?
The fall back is use standard M4 nuts with a dab of glue to hold them in place, would be nice to do it properly though.