You already have your masters license, if you have been sailing master and have all your STCW codes on your MMC, then you just need the document from an accredited school covering two modules. Some schools will do both modules during the same session, others want more money and drag it out. That way you get to waste more time as well as money! I am not sure, but I believe the.modules you need are labeled 206 and 207. They cover celestial and masters navigation and business. Again, some schools make this a two or three session class. Check them all out.
I went to CMTI in Hayes, VA. No BS, a lot of hard work, but no BS, 10 days and done.