Fin-Addict Marine (contact Brown,
sales@finaddictmarine.com) has the 265 dash templated for max surface area and has done several for folks on this site, mine included. The piece they did matched up perfectly with the outer line of the overall dash panel area and was enough for me to mount a Raymarine Axiom 12Pro and an Axiom 12 (2nd pic) though installation did require some cutting to enlarge the dash opening (1st pic). The carbon fiber ABS (3rd pic) was added for appearance, the 1/2" starboard panel is plenty strong to support the weight of the units. Believe the Garmin 12s are slightly taller than the Axioms and you'll definitely want to test the templates first but there's probably 1/2" of upper dash that could be removed, I only had to square off the corners to make mine fit. The 2 pair of holes just above the opening are from the original plexi door hinges. Don't forget to account for how deep the units go into the dash or how high off the surface they sit, things can get tight on both sides of the panel.
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