By the way, this is my first instructable so be nice! ;-D
Step 1: Getting started!
So... what to do? Just build one! It's really easy and effective plus you a reusing things you would probably throw away!
What do we need:
1.)_An empty bottle_; One small soft drink, water or whatever bottle with an screwable cap will do!
2.)_A clean plastic bag_; A normal grocery bag will do, just don't use a really thick plastic bag, it's worse for squeezing!
3.) A knife, cutter, scissors, anything like that.
Step 2: French Bottle Revolution!
Note that there will possibly be many pointy edges in the plastic. You should try to make the cut as clean as possible by cutting off with a cutter those parts or just use heat from a stove o lighter to make them blunt. Failing to to it will cause hole into the bag when you are squeezing it and all the content comes out.
Get the cap and with the help of a scissor peel the plastic circular seal that exists in the concave part of it.
Now bore a hole into the cap. I just used a pair of scissors.
Step 3: Making the contour mask
Finally put back the seal in place.
Step 4: Finishing Off
Final consideration: You could alway use only a bag with a hole, but I find adding the bottle head helps when decorating pastry.