I dont think though that he will be able to fit the washer or what ever he is using to hold the laser module in place will provide a sufficient tight water proof seal around the top of the torch.
Put a rubber washer between the metal washer and make sure its pushed up tight. Should be if the laser module is cut to exact length.
You can always just get some waterproof glue and squeeze it around the seam on where the washer sits to the body of the torch then wipe away the excess. Waterproof silicone is only 4 dollars a tube, although buying a tube for this tiny project will be a waste.
that bike light will probably be too small, I've got one. the switch is on the top and is momentary. So you might need to replace that switch with something else too. I think it would be too much trouble. the one from dx for 10 bucks looks like your best bet.
Yes I have the exact light, bought from a different ebay seller but looks identical to the picture. I bought it as a bike light, not for modification but looking at it I wouldn't bother with that one myself, the switch would be useless as a momentary switch in my opinion, you would need to change it to an on/off switch and mounting the switch would be a problem, you would have dificulty fitting everything in there too, its made of plastic and is cheap quality in general.