I have a couple of these MSR canisters bought specifically for camping (stove fuel). I've never used them for petrol, but have carried them with me on my bikes. They do have a very narrow neck, but are very well made and I wouldn't hesitate filling them with petrol and chucking them, upside-down, into a pannier.
I've never carried additional petrol with me on a bike, just not needed where we live! But I'll confess to nearly running out in the UK when I first rode the NC500 on my Street Triple. Tank range was around 180 (when behaving, which is kind of hard to do with such a twitchy throttle!) anyway the first time I completed that ride, 24 hour petrol stations were unheard of up in the Highlands. Plus it was a Sunday... Panic stations ensued as I was scrapping the barrel, but then I found a proper old-skool shop with a single pump outside and an old gentleman to operate it for you. I was so thankful! The next time I completed the ride, I was kind of sad to see 24 hour petrol stations (along with a sharp increase in traffic!)
Anyway, for my Europe trip, I only ran really low a couple of times, but that was my own fault. I hate stopping for fuel when I'm in the groove, so was pushing my bike to the limits. I was carrying a Givi canister around with me which is designed for petrol (but I had it filled with fuel for my meths stove. I would happily use it to carry petrol if we do end up on a 'YOC does Italy' trip. It has a 2.5 litre capacity, so very pretty decent for a back-up, and it never leaked. Here's a picture of it from Strasbourg (attached to a removable mount on the rear of my left pannier):
The canister itself is cheap to buy and didn't leak once. It comes with a separate hose which I didn't need for my stove. Downside is perhaps its built specifically to go with the mount (expensive) and again, really only designed for Givi hard cases... But I'm happy to fit more to my bike if required.
I do also have a small Rotopax canister, but I wouldn't waste your money! They are a great concept, but ridiculous money, and I've yet to use mine without it leaking, so wouldn't feel comfortable strapping it to a bike. Out of all my canisters, this is the only one I have used to carry petrol (when topping up at a station in my car and then filling my bike up at my lock-up). I do like the fact the hose is stored internally though...
For those who feel the need to take extreme measures... the president of the UK Iron Butt Association has gone to the effort of installing an auxiliary tank in place of his pillion seat on his Tiger 800 to reduce fuel stops...! I did have a twin tank fitted to my classic Mini (needed filling via it's own fuel cap). It was always amusing at busy petrol stations because you could see impatient drivers pulling up behind me because they assumed I wouldn't be there for long. You could see their regret when I then pulled the hose round to the other side to repeat the process...! Here it is right after installation (before I tidied it up):