Dropping 24V to 12V simply and efficiently
(Edited) I know this is a two-year old thread, but I still want to answer. Although, you have  eliminated DC-DC converters I still feel that's the best option. A buck converter will do the job in the blink of an eye. If you can't get one locally, order from one of the international shopping websites (EBay, AliExpress, Bangood etc.)

I recommend an LTC3780, since you want to handle 5A. It also comes with current limit adjustment and charge level indicator, if you need them in future. I hope this helps.