For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Marvin Reynolds
Not from the Bible but from Roman Catholic interpretation of how that sign of the Blood placed on the door of the home when the Angel of Death appeared in Exodus came at the end of the Curse on Egypt. This sign was one of protection and is recorded in Chapter 11 of Exodus. The placing of the Blood on the door was done in the manner of the CROSS.
Gary Wainwright
Exodus 12:7 teaches that the blood was put on the two posts and lintel of the houses therefore it was not in a shape of a cross.