Force4 are have a Fusion promo at present. This one is NMEA2K compatible and fits a standard DIN cut-out
https://www.force4.co.uk/item/Fusion/MS-RA70N-NMEA-2000-Marine-Stereo/9W7?utm_source=Force+4+Exclusive+Customer+Offers&utm_campaign=5abbbc5ebb-enewsletter_NL0621_08_Marine_Audio&utm_medium=email&utm_term=0_2ed54a55f8-5abbbc5ebb-129531157
If you're talking about running through the leg of the rail it probably won't be too hard to drill the rail foot and thread the cable inside. Depends if you have good enough access under your bow/pulpit to run the cable to the rail foot though.