Five pounds or five gallons? Not that it matters, but I have one of those five pounders, which isn't much bigger than a gallon milk jug and it's REAL handy! Also easy to stow.
If yours is in fact the more standard twenty-pounder ("five gallon"), you'll need a bit more space...
I agree with the previous poster re. little danger from overpressure leaks, especially since your cylinder is new and literally can't be filled beyond 80% of capacity. As noted above, transportation inside vehicles is allowed so long as the cylinder's kept upright. Most folks use milk crates for that.
If you're nervous about the milk-crate-in-closed-bay idea, you might drill some ventholes in the floor of whatever bay you put the bottle in.