I have a lot of information on my web site, as well as a photo gallery of what others have done.
If I have any suggestions, they are: Dump the mag mount! Drill the necessary hole and do things right.
Whether or not there is a hole in the firewall makes no difference, as one can easily be added.
One consideration about using existing DIN slots is heat. Not so much because of a lack of ventilation, but in most vehicles the slot is right in front of the heater outlet.