Cut a section of cardboard to the desired square or rectangular dimensions for your project.
Measure and mark even intervals to place your columns.
Mount paper towel tubes on the marked positions and attach to the cardboard base with white glue.
Measure the distance between two columns. Measure from the top outside edge of one column to the top outside edge of the next column over so that your measurement covers the diameter of both tubes.
Mark a line the same length as your measurement in the previous step. Find the center of the line and draw a line perpendicular to your first line that is two-thirds the length. Create a triangle by aligning your ruler with the left edge of the first line and the top of the second line and draw a line to connect them. Repeat on the right side.
Calculate the area of the triangle by multiplying the base times the height and dividing by 2. Divide this number by 3 and multiply the quotient by 4. This should be the area of your parabola.
Create the parabola by extending the triangle sides out to the arc. The simplest way to accomplish this task is to find a bowl whose rim aligns with the vertices of the triangle and tracing the curve.
Cut out the arc and trace onto a sheet of card stock. Make enough tracings to place a parabola between each pair of columns. Cut out the centers to leave a thin arch.
Mount each parabola between a pair of columns and attach with glue.
Complete the structure by attaching a Styrofoam roof and adding traditional Greek architecture elements such as grooved pillars or decorative caps from Styrofoam carved with a craft knife. Apply to the cardboard with white glue and paint the entire assembly white.