I have the same configuration of batteries, but with two parallel 12 volt systems with two alternators.  I built a battery box and routed a 1 inch SCAT hose to blow air from above the engine through the wall of the battery box into the slight gap between the batteries.  The battery box is made of left over 2-core-2 honey comb and fiberglass used elsewhere on the aircraft, and so is inherently a therma insulator against higher external air temperatures.  Been five years and had to  replace only one battery after three years, other going strong.

 

Fred Moreno