Aluminum is a weak metal, an it is probably not that thick either. You really want a bolt with a significant washer on the backside to dissipate the load of the extinguisher in a crash. It is just not worth the potential harm to take chances.
A 60 mph+ crash in one of these cars is dangerous enough, you do not want to have the good fortune to actually have everything work well enough for you to survive a high speed crash, except for being taken out by your fire extinguisher
