As I'm finding out these last couple of days if you leave the cover off the hatching trays you need to fashion a barrier across the back side of each tray to keep the poults from falling out and getting caught between the trays and the back of the incubator. I used some aluminum foil folded over several times to make it stiff enough and just short enough to fit but taller than the inside of the trays. Once I did that I then put the trays back in pulled forward far enough that they just touched the inside of the door so that they could not fall out that way either.
Just took some more out this morning for the trip down to the brooder.
.....Alan.