Building a low-power solar/wind buck-boost converter
You may not need a power supply at all, just a diode between the solar panel and the battery.  Lead-acid batteries can be float-charged for long periods of time without damage.  If the maximum current the panel puts out at about 13.6 V doesn't exceed the float charge current of your battery, then there is no need for any converter between the panel and the battery.

If you're worried about efficiency, save the time, effort, and money that would go into the switching power supply and get a bigger solar panel instead.  Sunlight is free, so efficiency is more relevant measured in effort or dollars per Watt than percentage of energy conversion.  Get a solar panel just big enough to deliver the maximum float current at 13.8 V, then put only a Schottky diode between the panel and the battery.