For the moment I have just used the oem subwoofer wheel nut to secure the tool tray in place. But if you wanted, you can reuse one of two front anchor nuts, use any collar M6 nut, or use OEM part N01508215
I don't understand why the hole for the nut wasn’t predrillled especially when there is an OEM nut listed to secure it. May be a difference in variation.