Bumper brackets
Washer
NSPIN6X18BLK - Spin Nut Black Oversized Washer
I think I used captured washer nuts and a closed end wrench to hold the nut down in the trunk extension while starting the nut by hand from the outside. IIRC...? This was done while bumper was off and I left the bolt loose until the bumper side bolts were in place.
I think this is how I did it...????