Is it possible to use a motorbike filler cap? Most of them seem to have nice "aero-style" flush fitting filler caps, and are probably much easier (cheaper) to source than the dedicated car equivalents.

Are am I missing something? (you probably need a seperate funnel, but you have to buy that seperate anyway, right?)

That is exactly what I planned...I have already bought a used bike tank, cut out the filler neck, bought a shiny chrome cap and new keys, will connect with flexible hose to tank. also, may use curved side of tank as an engine bulge in bonnet, as suggested by some nice person here, cant remember who

Picture please!