First I had to use my Styrofoam knife to cut the shape and kind of dig out a space for the support PVC to go. Now using a Styrofoam knife can be a bit stinky so make sure you have plenty of ventilation.
While the glue was drying I placed small blocks of Styrofoam under the arch "legs" to keep them straight. This pic is just before I added the existing front of the archway. You may wonder why the lower part of the PVC is painted orange. This is because that part of the archway is inserted into carved pumpkins.