Hi, try this!
I Initially removed the fuel/adblue filler surround to see inside (let it hang on the overflow pipe, don’t detach it!). There is at least 60mm clearance behind. Find the correct position and use the bracket as a template (I removed the carpet beneath the bracket area to make installation easier). First drill 4 pilot holes (3mm) but make sure the drill can go no deeper than 60mm (I used a collar on the bit). The outer holes (nearest door) need to pass through 2 metal skins about 10mm apart. Ensure that both are drilled through. I used 5mm rivnuts which required 7mm holes and are 13mm long. Again, drill the holes using the pilots but not exceeding 60mm deep and again going through the second skin.
For the rivnuts I purchased an installation tool from eBay (about £11, it’s just a hardened bolt, spacer and m8 nut -
M5 Engineer Heavy Duty High Tensile (12.9) Rivnut Insert Tool Nutsert ** | eBay). This ensured the rivnuts were correctly seated. I was able to view one through the filler hole and check correct rivnut install. For fixing I used stainless M5 countersunk socket heads (2 x 25mm, 2 x 35mm). The spacers were 8mm that I had lying around and seem to do the job.
Hope this helps!