This isn't a flashlight, but f you're a complete DIYer, you could use 3/4" PVC. The difference in diameters between the pipe and the batteries is about 2mm, so they should fit pretty well and if they didn't, it wouldn't be hard to stuff something in the pipe to stop them from shifting.
You could install a switch in a screw-in/out end cap.
Daedal's circuit would fit into it, and you could even possibly drill a small hole and fit one of the pots from Digikey that Daedal linked in his driver thread. That way you could adjust the power without needing to open it up.
To finish the top, you could use 1" copper pipe, which would double as a heatsink for the module. The OD of 3/4" PVC is 1.05", so it would fit snuggly with a little bit sanding to the PVC. A 1"->1/2" reducer coupling could even be used to fit closer to the Aixiz module; you'd just have to cut a piece of the 1" pipe to fit into the end of the coupling so it would fit on the end of the PVC.
It would take a lot of work, but it might end up being a pretty cool laser.
Thanks, Daedal, I've kind of been thinking about making some sort of PVC set-up for a while now, but hadn't put too much thought into it. Larry's post got me thinking I figure this sort of laser would cost about $20-$30 for everything but the diode and module, which isn't too much, and it would be a pride thing if you completely made a laser from scratch ;D