Filing N-445 is a 5 minute job. I did it while I waited, as well as many others on that day. Practice now http://www.ilw.com/forms/N445.pdf , but don't sign at home before you actually present to the ceremony.
But how will the securoty guard let you in if you don't have an oath letter?
By making an inforpass appointment? or just explain to security guard you
lost your letter?