Battery boxes

I would like to move from a rack mount, 3 battery system to a frame mount battery box with 4 batteries.

There are two water bottle mounting points, one on the seat post and one on the lower frame bar (don't know the right term for it). There could also be a strap across the frame and bolted down to the box near the top of the box.

I stacked four 12aH lead acid batteries in the frame, they fit although there is very little extra room at the top, which may be a problem. They fit in so that the narrow dimension is the dimension between the pedals so there should be no pedal interference.

What is a good material to make a battery holder from? My first thoughts are straps of aluminum which are clamped around the batteries and tightened with a bolt. There may need to be additional straps around each battery.

A wood box is another possibility. I am looking around for old metal cases from computer and electronic stuff but the chances of one fitting exactly are pretty slim. Anybody have pictures or examples of successful battery boxes?