is it possible to make my own petrol tank out of carbon fibre ?

Ive read a few bits on tanks before. 

One bloke built a bike tank using the "lost foam" method and hand lamination, then using acetone to dissolve the foam afterwards.

There are quite a few tank sealers available in the classic car world that get used on ropey old fuel tanks.

Also motorsport often use a kevlar bag tanks in a composite box.