How do I avoid overcharging a battery?
It doesn't sound like you need anything more between the solar panel and the battery than just a Schottky diode.  Your panel is rather wussy compared to the battery, so it doesn't look like there is anything it can do to hurt the battery.

How much current can the solar panel deliver at 13.6 V or so?  That is usually the voltage that "12 V" lead-acid batteries can be float-charged at indefinitely.  The panel probably can't even put out enough power to get to 13.6 V, especially with a diode in there.

The real problem may be that the panel can't produce enough voltage to charge the battery all the way.  Check the panel and battery specs carefully, and don't forget to consider the voltage drop on the diode.