You could go buy a foldable (and prettier) portable dog water bowl for $10-$15 or you could make one in less than 30 minutes and do some recycling too.
Step 1: Materials and Tools
Flexible Plastic Sheet at least 28cm x 28cm
Piece of scrap wood
I am using some housewrap that I got from the throwaway pile at at local construction site. It feels like fabric; is light, watertight, and strong. It is used in the same way that Tyvek is. (I do not know if Tyvek would work for this project).
And the following tools:
Step 2: Measure, Mark, and Prefold
2. Make a mark 7cm and 8.5cm from each corner. (these marks will be your folding guides)
3. Cut out the square.
4. Use a straight edge as a guid to make a fold. Use the marks that are closest to the corner, and fold inward. Make this fold on all 4 sides.
Step 3: Plastic Welding
1. Heat the soldering iron
2. Place aluminum foil over the area to be welded together.
3. Move the iron over the area where you wish to make your joint, melting the pieces together.
I use the clamp to hold my piece of scrap wood which is my welding surface.
Practice on some scrap pieces. Use the flat side of your soldering tip. Also fold the Foil in half in order to minimize the chance of fouling the soldering iron with melted plastic.
That is the basic technique. Now we start by tacking corners.