A DIY system to avoid having your mask confiscated during a demonstration. A way of being effectively protected against tear gas, provided you master a breathing technique.
You have to breathe in with your mouth through the pipe, so that the air passes through the filter, and breathe out through your nose (without spitting air out of the pipe)

You will need a P3 type gas filter cartridge (most often A-P3), strong and reinforced tape, a cutter and a pipe that must be cut to the appropriate length.

1) Open the front of the filter and take the plastic plug
2) Make a hole in the plug using a cutter, to pass the tube and clamp it with a screw
3) Tape the whole abundantly, with tape covering both sides of the plastic cap
4) Add a rubber band and fix the cap on the filter.
5) Use the filter while breathing with the right technique

Remember to unclog the bottom of the filter when you breathe, and to plug it back on when you have finished using it.
A glove placed at the end of the tube can be adjusted and prevent air from passing through the filter when not in use.

How to craft a homemade gas mask

Cut a transparent 2 Litre soda bottle as indicated
Glue a strip of rubber foam on the inside edge of the bottle
Glue a new strip of cloth over the foam rubber
Put a clinical mouth-cover in the neck of the bottle
Elastic to secure it to your head
Soak the mouth cover in vinegar before putting on the mask