Photo of the insulation and joint tape used below. Just the standard household type for back of rads?

For fixing Just replicated what Autohomes had done originally. Screwed down through plywood into floor through top of ribs. Every third rib in floor had line of screws. 5 rows by 5 screws total of 25.
Laid the insulation on top of lats you can see in previous photos and taped joints with silver tape. Then placed plywood on top of this.

Could not find any csk self tappers so ended up buying these from local DIY shop. Although these are wood screws worked fine. Drilled though plywood and floor of van with 2.5mm drill then just fitted screws with electric screwdriver.
Hope this helps.