As you don't need to bend the frame for a square or rectangular arch, I would suggest that you use sturdy material, such as 1/2" PVC or steel conduit. To give the arch some extra support you could tie monafilament line to the top of the arch and secure the other end to a hook in the ceiling. You will also need a base, similar to our balloon columns base, on either end.
The video below shows how to create the frame for a large balloon arch. It's specifically for an outdoor arch, but as you are planning quite a large arch, the tips should be helpful for you, too.
Hope that helps and good luck with your balloon arch!
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