On a recent longer ride in a rural area, I put an aluminum 1 litre gas can (like a water bottle) in the top box.
A few hours later, nearing E w no gas stations around, pulled over to get the gas can.
The screw top had vibrated itself loose and there was a puddle of gas in the top box.
Can was still 2/3rds full, got enough fuel to get to a station.
But the top box and everything in it reeked of gasoline. Thought it was going to be a $200 mistake,
as everything I read said gasoline smell cannot be removed from plastic.
Let baking soda sit in the box for 3 days, then rinsed it w hot water. Smell pretty much gone.
Put a few pieces of lump charcoal in there for another 3 days to be sure, can't detect any odor.
Now I screw the cap on extra tight and carry it under the seat where it vibrates much less.
Put a neoprene wrap around it to protect from whatever heat is generated under the seat.