I replaced the entire fixture and filled the screw holes instead of finding an LED lightbulb that would fit. I'm sure that the stick on 12V LED light bars that I used are cheaper (2 for $5) and light better. I never got around to posting pictures, but I'll run through the modification. It sounds more complex that it was.
After pulling the old fixture, I replaced the electrical wire. LED take so little juice that I ran telephone wire (AWG 22?). I used a light beige color that looked a lot nicer than the original black conduit. It was actually just a piece of an old phone extension from the junk drawer. I stuck the new LED light bars (they come with 3M double-backed tape) on the top of the side window frames facing up. They are the same width as the aluminum window frame and disappear. (Check to make sure that these on EBay are the right width and waterproof.) As automobile fog lights they would be a joke, but they worked perfect and give plenty of light in the 16 cabin. Because I used telephone wire (CAT 4), I still have some extra leads that I could use for a red LED light bar for night vision, sticking it on the underside of the passenger window pointing down.
http://www.ebay.com/itm/2x-Super-Bright ... 43ca9831e2
Because they are further in the cabin than the original fixture, they give much more even lighting up into the berth. In fact, I can sit in the berth with my back against the passenger side hull and read at night.
Much better than the original cabin light. I liked them so much that I put one up under the lip of the cabin roof in the cockpit so I now have a cockpit light. Plenty bright and almost no draw.
I routed the wire through beige stick-on plastic conduit (and filled all of the old screw holes), also routing it by sticking it to the aluminum window frame. Some of the conduit routing is pictured in my photo album under "Modifications-Insulating Coating" where I redid most of the exposed wiring in the boat. It has held up very well and blends in nice. It makes the cabin of the CD 16 feel Huge. HUGE.
Mark