One other idea is using a pair of high efficiency flashlights as spreader lights. In the battery comparment you'd put a small DC-DC converter to drop 12V to 3-5V depending on the flashlight chosen.
High end flashlights have the latest in LED technology (usually its a generation or two ahead of other markets including marine), are rugged, weigh almost nothing, and I think the spread of light would be just about right to illuminate the decks from spreader height.
This example puts out 174 lumens with 700mA at 3V. That would be only about 0.25A, 3W of 12V input
Quark 123, S2 Edition 4Sevens.Com
As others mention, this and any other ideas assume you can't find something suitable off the shelf. And I agree, you'd prefer to only do this in non-critical applications (currently my spreader lights don't work, so I think that applies) and have a backup. That said, this site supports a lot of do-it-yourself work in areas that are arguably more critical than electrical.