I don't think it's a LPG filler as some have suggested. It appears to have a locking petrol cap and everything has been sprayed (poorly) black.

I did one of these a long time ago for a mate and we had the taillight pop out on a small gas strut and then tilted down about 30°. This was done on a HG Wagon.

As I mentioned last night in the shout box the vapours and the electrical worry me. But I'm sure it can be overcome with some smart thinking. The whole concept is very cool on a custom and I encourage you to live the dream. Just don't ruin a rare desirable Sandman.