Yeah, they are only stuck on during compliance, and some shops are known to do dodgy jobs. Iv'e heard people with similar probs.
Best would be just some strong strength glue or adhesive that is of course petrol tollerant. Silastic gunk would probably do the trick (bunnings). There is also the stuff that is labelled as "petrol tank repair" which is a sort of putty which you can knead and dries to a solid seal.