Aluminum is not the best material to use for a battery box since the battery's electrolyte, dilute sulfuric acid, readily attacks aluminum. Mild steel would be a more suitable material. Most auto battery compartments are fashioned from mild steel - and still corrode, as you all have noticed, after the paint wears off! If you're going to use AGM batteries, then aluminum should be perfectly fine since there will be no battery acid leakage from these sealed batteries.
If you can't or don't want to fabricate a steel (or stainless steel) battery box, constructing a fiberglass one would be an excellent alternative. This method would require the building of a simple, wood male mold (much easier to lay up cloth around a male one than inside a female one) that is coated with wax before you apply the first coat of resin and cloth. Suitable epoxy resins and glass cloth are available in most hardware stores and, if carefully made, the box can be extremely strong and impervious to battery acid. Also, it may be feasible to cut down your present plastic box, add an aluminum or wood flange to it and cover the entire thing with layers of glass cloth and resin. Be sure to wear disposable gloves to protect your hands from the epoxy resin. Some individuals can develop dermatitis from exposure to the epoxy resins. :
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