I have not seen this in person so I cannot endorse the product, but a quick search on Amazon found this:
https://smile.amazon.com/UPGRADED-Interior-MIHAZ-Universal-Motorhome/dp/B07JNYBMZY/ref=sr_1_24?crid=SV5RHE7MYZSE&keywords=marine+cabin+lighting+fluorescent+replacement&qid=1644342693&sprefix=marine+cabin+lighting+fluorescent+replacement,aps,60&sr=8-24&th=1
Well, I started with this option, and ended up ordering this one:
https://www.amazon.com/dp/B07DQSX4TN?smid=AXEJX2LGR9968&ref_=chk_typ_imgToDp&th=1
My thinking was that I either find a fixture that fits inside the indention above the medicine cabinet/mirror, or one that completely covers it. I opted to cover it. The fixture is 26" long, which should perfectly cover lengthwise. However, I didn't bother to measure the width/height, so my only concern is that the fixture would be wide enough to awkwardly extend past the angle in the ceiling. We'll see.
The only other modification will be paint. I ordered new reading lamps for both sleeping quarters, and the boss decided black fixtures looked more modern, so I'm planning on taping off the lens and painting the light after confirming the fit.
Thanks for pointing me in the right direction, Harold, much appreciated!