I made my own for a sliding glass door. Bought the lamenated panel from Bunnings (comes in a range of colours) about $30. Cut to size. Cut appropriate size hole in bottom with jigsaw. Then get some stiff plastic, again from Bunnings only a couple of dollars a metre, and some hinges. Cut to fit hole and attach with small nut and bolt. I have added a strip of extra stiff plastic along the bottom to stop it from curling. Fit into the door and then use a thin piece of wood offcut to jam into gap at the top. This will hold it in place. You can still open and close the sliding door as it just butts up against the edge of the dog door.
Good luck!
