Here is a favorite that can be used in many different ways. I used a discarded cardboard grid from a game, but you can use a carpet knife on cardboard from a box, scissors cutting a cereal box, or make your grid from card stock paper or Matte Board (used for framing).
Some skills that were addressed in the pictures shown are; visual-motor, visual task attention, fine motor strengthening, thought organization, spacial awareness, cause and effect, auditory processing, visual discrimination (I told the kids that if they used a light color, then they had to choose a dark color for the letter), writing practice, and following directions.
Option: You may choose to have client write within a square (using the grid) and then write in a square shape that is only in their mind. See how close it comes to looking like a square (you can have the grid next to them to refer to).