Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I feel fairly sure that you have mis-stated your model #, unfortunately: the 8800F is a flatbed scanner. It has no 'rear panel' to remove, it's the wrong shape for that.
You must be asking about a printer. (After all, you say '8800F printer'.) Doubtless the rear panel, at a guess, is actually a duplex unit. Usually (in a Canon printer) those are put back into place by first seating one side in place, then holding down a catch or push-latch while fitting the other side into place, then releasing the push-latch so that it holds the panel or duplex unit in place.
Please determine your machine's actual maker and model #, and let me know, so that I can give you exact instructions and for your exact model.