If your heart is set on a solid mass of flowers, I find one of the easiest ways is using Oasis Sculpting Sheets. These are large rectangles of florist foam glued to a sheet of Styrofoam.
These sheets are about two foot tall and a foot and half wide. Fairly manageable to transport. Keep in mind that they can get heavy after saturated with water and filled with flower heads.
As you can see - the Oasis foam is pretty thick. Since it is awkward to soak these on site, I typically soak them in water treated with Crystal Clear flower food before transporting, then attach them to the archway on site (especially if it is a permanent structure and quickly flower in.
These can be cut into smaller panels - but the stryrofoam can be messy and cutting is best to do BEFORE transporting.
To attach to the archway, I insert Cowee Stakes into the Styrofoam layer at even intervals (helps to coat with white glue) as shown below. You will use the stakes as anchor points to wire or ziplock to the archway. (This was demonstrated with attaching the raquettes and spray bars earlier).
These sheets can be VERY HEAVY after saturated with water (and especially after adding flowers). It is important that the sheet has a lot of sturdy anchor points and is attached firmly to the archway. You do NOT want the arch panels to fall off during the ceremony!
If the arch is a lightweight structure, I would recommend using individual cages of foam rather than sculpting sheets, as the heavier sheet could possibly buckle or collapse a lightweight, cheap archway.
After you attach the panels - you simply cut your flowers and start quickly inserting into the foam.
I really do urge you to consider individual cages or raquettes. Their smaller size makes them much easier to attach and then work quickly in flowering the design. Time is often of essence when working with fresh flowers and I find the sculpting sheets much harder to handle than individual cages with predrilled holes for hanging.