I have the $140 Rally Armor flaps, and after installing them, its really not complicated to be able to make your own. There isn't much to template.
Inside the wheel well, there are plastic screw tabs holding the wheel well liner to the car. Get a large piece of paper or cardboard. Place it in your wheel well how you would like your mudflap to sit. Mark the points where the plastic screw tab fasteners are for your outer mounting points.
The inner part of the flap is mounted to a bracket that attaches to the car at another one of those plastic screw tab fastener points on the underside of the car right behind the wheel well. This bracket reaches from there to the inner portion of the wheel well.
Now you have your template on you piece of paper. Copy this to mud flap material. Cut off excess where it is not needed inside of the wheel well.