Apologies for the poor description but the part number for the plastic is MB641193. My car doesn’t have the aero whatsit and so the bottom tray is in three parts. I’m after the bit on the left.
Check out Amayama. Some parts not available but there could be something here.
https://www.amayama.com/en/genuine-catalogs/epc/mitsubishi-japan/gto/z16a/mnpm-5296/body/610