Just something to think about...... if you get hit hard enough to break open a gas container built to handle a lot of abuse, then it is not likely that you will land anywhere near the bike, or the container of fuel. If you can ride with a container of fuel between your legs, then don't worry too much about one strapped on the rack. It might be a problem if you were enclosed in a metal cage and strapped down to the seat. If you are sitting straddle on a machine, and not tied to it in any way, then not so much...
Totally agree with this. There are a LOT of other issues to be worried about if you get hit hard enough to break open a gas container.