Here we go
http://www.vanjast.com/NavMod/RealNav_Sextant1024.zip
I cannot test this for accuracy as my screen is 1024x768 , so it's up to you to check it out. Please read the text file as it has the info required to manually insert this. It simple enough so should not be a problem.
To test this - make a simple single mission in the mission editor.
Place your sub at 60 North (anywhere), with flat seas and clear sky, 00:00 hours.
When playing this mission, find Polaris and if it's on the top edge of your screen (with bottom of screen aligned with the horizon) it should be ok. this will indicate that the 1280x1024 screen is nothing more than a scaled 1024x768 screen..
Happy Nav (like 'Happy Feet')