Here is my head unit going into honda OEM nav trim. The fit is pretty good. I just need to devise a little trim ring to fill the gap around the edge. This is going to be perfectly rectangular so it doesn't have to be nearly as involved as previous pieces (hooah!)
now I cut that rectangle with a little bit of draft on all sides so it will drop into the opening. Straight sides won't go in right, so if you're duplicating this, all you need is 1-2 degrees of draft.
nothing like a perfect fit ;-)
and then cut out the opening for the head unit. Takes a little finesse but not too much trouble.
everything goes in pretty easily.
and finally, a little flat black paint and life is good...enough....for now...
Not that i ever questioned it the way I'm doing it now, but i just wanted to fire up the unit and make sure all systems were go... and that I could adequately interpret the installation instructions... and for the pretty lights...