By drilling 4 small holes on the sides of the Tactical Rail, large enough to fit small zip ties, you can loop the ties through the tac rail, around your flashlight, and secure them tightly together. If you use a flashlight that has grooves in it (like this one), it's bound to be more secure than one that has a smooth grip on the outside. Find a flashlight that'll do the job and you're good. It also might be helpful to use a flashlight that has a push-button activation instead of the traditional MagLite approach with a twist-on head. It'll be easier to operate during games than having to manually twist the head to the right setting.
NcSTAR DP Red Dot Sight. These, while designed to fit onto weaver rails, sit higher on the rail and can actually adapt to slightly different sized rails. You can mount it securely to your N-Strike Rail with a Philips Head screwdriver and they usually run from 8-10 bucks. Pictured to the left is my friend Eleri who's got his mounted to a Phoenix LTX. The LTX's rail is actually WIDER than an N-Strike Rail, and it still manages to fit on there. The only reason I don't have any shots with a Nerf Blaster for this example is because they're really not that useful when you're lobbing foam darts. Line-of-sight targetting systems like scopes or sights are only useful for line-of-sight blasters (like laser tag). Still, I know that folks like to have that "Tacticool" look to their blaster and if you have a modded blaster that can hit stuff from far away, this could still be useful to you.