Here's a suggestion:

Buy a green backpack, 5' of 1/4" OD clear tubing, and a can of yellow spray
paint.

Make a stencil that reads
"OXYGEN
NO SMOKING"

Stencil these words onto the backpack using the yellow spray paint. Duct tape
the free ends of the tube to the inside of the backpack, but leave a loop of
tubing hanging out of one of the zippers, os that it is visible.

See how much fun you can have!

Might work... might get you arrested.

Harry