I'm the opposite, it looks like a solid idea to me. I've never had a cop try and inspect my cargo and I've never put myself in a position where they want a warrant to search my car. If it's to that level of a stop, I probably have other things to worry about instead of having fuel in a non-DOT approved container. As for having the top open and a spark from the camp fire hitting it, I figure that if I have 8 gallons of exposed fuel next to a campfire, I probably deserve whatever comes to me.
Not totally discounting the dangers, but as someone who's been in several countries where any container that doesn't leak is used as a fuel can, this looks to be a much better container. Depending on the price, this would be great as a fuel container to lay on the roof rack in the horizontal position. To do that I'd probably weld it shut, and install a cap and a vent and away we go. With a nipple on the side and a hose, it'd make roadside fill-ups a breeze and you'd never have to take them down.
How much does a 81mm can go for these days?