If the original plastic tray is broken in the fixing points where the clips came out, you can replace it with our aluminium version, see here:
https://www.torqen.uk/nissan/370z/body-styling/exterior/1664-370z-torqen-undertrays-trq-ut370.html
If you need just the bolts and nuts, see here:
https://www.torqen.uk/nissan/370z/body-styling/exterior/9491-370z-torqen-undertray-hardware-kit-trq-ut370-hk.html
Very, very easy to install / re-fit, you need a 10mm socket on a small drive
More information, pics and install guide here: