Here's a photo of the bare metal plate assembly that goes inside the car. The 3 bolts are round flat head machine screws 1/4-20 x 9/16 spot welded to the plate. The hole in the center of the plate is for a Phillips pan head tapping screw #10-12 x 1/2, zinc plated.

The plate and bolt heads are hidden behind the back seat, so hex head machine screws could be used. If you need to make a plate assembly, it is simply sheet steel 0.056" thick with 2 bends. PM me for a drawing or more details.