Lawrence, you say you want to reduce the supply to 24 V but as the lights work from 10-28V why not use a lower figure? If your supply goes right down to 24V or even less (I'm thinking a yacht with solar power after several gloomy days) then it will be very difficult to regulate to 24V. And the lower the input voltage the less likelihood of the lights failing.If your lights will work at 12V (they should) then regulate down to 12V. You can get small switching regulators that are drop-in replacements for the well known 7812 but are much more efficient (an important consideration here). Here is an example:http://www.gravitech.us/312v1aswvore.html
They can supply max 1A - ie 12W - so theoretically can power 3 of your lights, but I'd stick to one per light or maybe one for 2 lights. They can take a max 40V input so no problems with your voltage swing.These are $9 each so would cost you a bit for 10 or 20 of them, but you should find some cheaper than this if you look around.
