I (and many others) have ferried a light single (Mooney most recently) with a long range tank taking the place of the aft seats. Much more than 20 litres, and I can promise you that a jerry can is a lot more robust than the ferry tank.
That said, of course it was properly installed (though not bonded, I think). So in normal circumstances I would support the view that carrying a jerry can is not the best of ideas if it is not absolutely necessary.
(But then again I can also tell some stories about flying with my feet resting on a can of AVPINIsopropyl nitrate - Wikipedia, the free encyclopedia