Sorry to wee on your potential bonfire(couldn't resist that one) but I think you'll find that carrying fuel in any part of an aircraft not designed to do so is illegal.
Having said that, you can get temporary ferry tanks fitted to some aircraft for temporary use.
As regards jerry cans, I think some people in the microlight/permit aircraft world do sometimes do this (still illegally) and get away with it, generally by strapping a jerry can into the passenger seat.
As regards the dangers, I would guess its as dangerous as you make it. I wouldn't do it but I wouldn't necessarily condemn someone who did. How well will it be secured? Does the can have a secondary locking pin for the lid? Are you in an open cockpit so fumes aren't an issue?
Edit: Apologies, I've just noted what country you're posting from and I'm not sure on the regulations governing this issue there.
As regards practicalities, could you let us know what the purpose is and what the aircraft type is and then perhaps we could be of more help?