The placing of a Christian symbol on the coffin is optional, but if done, OCF 134 has a brief instruction on it:
134. If in this rite a symbol of the Christian life is to be placed on the coffin, it is carried in the procession and placed on the coffin by a family member, friend, or the minister at the conclusion of the procession.
Logical. Put the object on the coffin after it has been moved into position.