How do you make an effective Wing out of a sheet of paper, or a sheet of plastic foam? You can leave it as it is, cut it into a pleasing shape, and put a power source on it big enough to force it through the air at a speed sufficient for lift. Kites have this shape, with wind for power. You can form the sheet around an object so that the sheet takes a permanent curve, called an under camber. The wings on the Wright flyer had this shape, and they certainly preferred it to a flat plate. You might form your sheet into a recognized efficient airfoil, maybe a Clark Y section, as used on the Spirit of St. Louis or a Piper Cub. You might carve or mold a thick sheet into the section, or form a thin sheet around ribs and a spar. Both these methods require more work, and add more weight per unit area, than just cutting out a flat plate.

There is another way to make a wing out of foam sheet, that was discovered by Dick Kline and Floyd Fogelman when they were looking for more efficient, i.e. longer gliding, paper airplanes, now called a Kline-Fogelman airfoil.  Folding a paper airplane makes a wing with a step, and Dick and Floyd found that they got better performance with the step in the right place.  Since then, when R/C modelers discovered blue insulation foam sheet or foam core poster board made a good combination with clean electric power for building lots of fun planes, their discovery found a home.

         
                          http://theparkpilot.org/kline-fogleman-airfoil-design                        

While adding an extra strip to a flat plate foam wing might be justified as a reinforcement, or possibly a way to increase the critical angle for stalls, others might argue that Kline-Fogelman wings don't live up to expectations. This doesn't deter enthusiasts from building planes that fly with such wings, and doing it with a minimum of fuss.

                      https://en.wikipedia.org/wiki/Kline%E2%80%93Fogleman_airfoil 

My Gym Stick has an under camber airfoil, probably best at low speeds, and a thin K-F strp, probably just a reinforcement. My EZ-Fly has a more prominent K-F strip, and flies just fine. My NutBall has no K-F strip, and might be a more stable flyer if it had one. I surely would have used less glue repairing its wing's leading edge.