Sorry jviss. Not trying to harass you, just answering your question "What material should be used, and where can I get it?". Hunter isn't the gold standard but MainSail is highly respected here and he appears to use wood. Wood is very common and appears to work. Could your batteries be boiling over ?
I do not use bare wood for flooded batteries. Flooded batteries reside in an acid resistant box with no metal screws-in the bottom. AGM & some GEL batteries, classified as "non-spillable" are a different ball of wax because they can't spill or leak acid.