First thing.......... if you had a contractor in that found the combustion chamber out of position or damaged and still fired the furnace.......... find a different contractor! A contractor experienced with oil fired heating equipment will know where to find the combustion chamber and needs to be the one installing it.
The fiber combustion changer is there for a couple of reasons, the most important one is to protect the metal heat exchanger from damage from the extremely high temperatures produced by the flame. You should not be running this furnace without the combustion chamber in place as it was designed.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.