replacing bolt on rear access flap...

[blue]

I have a bolt missing on the rear access flap to the engine bay [the one where you top up oil and water]. I want to replace it, but can't get one off my other van to try. I've had a look at other options, but I have this feeling that there's another part that goes behind the metal panel [nut? clip?] for it to bolt into, as I can just feel the hole in the bodywork when I poke around. Anyone know the part number for this? or an alternative solution [big self-tapper?]? Maybe you have one lying around?