If you can't find a use for this, then you obviously have not been in a situation where you've wished you could pee through your fly... If the thought offends you, then please do not read on.
Step 1: Materials
What you need:
Plastic cutting board (the pliable kind)
Two pieces of wood (I hear thick books can also work)
Step 2: Use the paper/carboard to make a stencil.
Fold the paper in half and draw a curve across one edge. (the crease is on the bottom). Next cut along the curved line and unfold your stencil. The length of the STP device is up to personal preference.
Step 3: Use your carboard stencil
Step 4: Melting the edge
Ok so please do this step outside as there are likely fumes created from melting the plastic. Get the assistance of someone else if you don't think you can do this safely.
Place the folded plastic piece between the two pieces of wood. Place it so that the curved part is poking out slighting from the wood (a few mm)
Now i was unable to do this for the second photo (as I was holding the camera), put pressure on the wood so that the plastic parts are together.
Use the lighter to melt the plastic together. Don't stay too long in one place, instead wave the flame gently back and forth for about 30 seconds. The intent is to seal the plastic not burn it. When the plastic is hot use the wood to press over the hot seam in order to seal it properly.
In order to get the full curve, you will likely have to adjust the plastic and melt the rest of it. Take your time.