No not that I know of.
The template that is in the back of the manual is 138mm x 73mm. Roughly 5.5 inches wide x 3 inches tall. Use the template when Marking the cut as the rounded edges and precise measurements ensure a perfect fit.
unfortunately No, not the GX1800 looks like it lacks the internal programming - the GX1850 is NMEA 2000 per page 79 of the owners manual.
I do not know, you may want to ask your local law enforcement.