Suitable adhesive to join two lexan housings

As an eletrical engineer, my chemical knowledge is rather small. I do the electrical design of a product that is housed in a small lexan housing and used in the power industry. The enclosure houses a circuit board, battery, LED, and flag indicator. The problem is that the enclosure cannot be opened so when the battery dies, the unit is worthless. Rather than redesign the whole product I was thinking about just drilling a hole in the enclosure, feeding the battery wires through the hole, and attaching a small battery housing to the outside of the enclosure. My question is, what would be the best adhesive that could join these two housings and could withstand years of being in the elements all over the world?