In addition to glue, there are a couple of other options, depending on what you prefer. One is to use double-faced tape to hold the case halves to the batteries.

Another is to take a soldering iron whose tip you don't care about and just melt the case halves together. Do this in a well-ventilated area; the fumes can't be good for you.

I'm too lazy to do any of these things any more. I just leave the halves unglued, and shove the whole thing into the 'book and leave it alone.