I'd rather encourage you guys to not attempt this little toy.
First, I doubt it's legal and with all the terrorism laws and stuff going around, you could find yourself in a load of trouble just for having it.
Ya, your friends can keep a secret, for about two minutes after they get out of your sight.
Second, this guy is using alcohol.
If the fire did leach back to you, your backpack is a bomb waiting to happen, especially if you have even the smallest leak, or spilled some on you.
Because alcohol can burn from the fumes with an almost invisible flame in the light of day.
Have you ever seen a nascar car in the pit catch fire??
It's fast and hot.
A real flamethrower uses jellied gasoline, it throws the stuff out in a stream, and as it disapates it becomes flamable.
AWAY from the user.
Alcohol needs only a small amount of oxygen to become explosive.
Close to the user, if not ON the user.
Why I just told you that, I don't know, but you realy REALY should not make such a dangerous toy.
I'd rather see you guys able to post than lie in a hospital bed with third degree burns over 90% of your body.
If your going to build it, use it for water, then you'de have the ultimate Super Soaker.