After building a PVC pipe battery holder for my electric bike, I used a vacuum form machine to make a polystyrene battery cover. I built up a cheap tool form using cardboard (which turned out to be too week, but it was quick).
As the vacuum was forming the polystyrene around the cardboard the box started to deform so I had to release the vacuum before the styrene had fully formed itself to the PVC pipes, but it got close enough to be screw mounted.
This cover will protect the batteries and a lot of the wire connections from rain, the weather, and prying fingers. In addition to the batteries, the cover protects two of the three fuses on the bike and a switch to control the lights.