Ive had a look on the search and cant find any info on the subject,
So has anybody fitted/tried to fit a motorbike filler cap?
They are so cheap but i dont think they have a hose tale on the back.

Tommy

your right they dont have tails on back they are part of tank.
The only way you could use one would be to get a damaged tank and cut the filler out and adapt it, but the breakers may not be too helpful in giving you the key as they sometimes sell complete locksets, worth a try thou!

You can get a cheap filler cap from a boat.

I did it and it worked out fine, as Hector says they don't usually have the tail but that is not hard to sort as for keys that is no biggie as the locks often only have a couple of pins. I bought a smashed up tank with cap and key from ebay and asked the seller to cut out the bit around the filler and send it to me (to save postage) check my blog to see how it turned out.
If a breaker does not want to sell you the key ask to borrow it and have a copy made ...