You could go the hella cheap way that I would do:
-buy a green laser pointer from a store or online for cheep
-get a bunch of cable-ties
-make some groves in the laser casing wide enough to groove a cable-tie
-cable-tie it up
But that's just me. If you have the tools, you could make something to attach to the gun to hold a laser.
Either of the two would present a problem, turning it on.
Best bet, replace the push button switch with a small, flat button and attach it with a lengh of wire. To protect the new push button, use heat shrink tubing (will at least make it more durable).
But, then again, this is my cheap way of making a laser sight for anything I need it for (potato cannon, airsoft gun). Don't know what the gun looks like so there might be some problems regarding securing it in a way that won't interfere with normal operation.
Wait, if it's a 90mW UltraFire WF-501B Flashlight-Style Green Laser (CR123A), then you'll have to put something under the thinner side so that it's level. Half circular metal pipe or wood maybe. Really depends if you want it to look pretty or just to work.
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On second thought, if you want to go full out and mod the gun (this is me just thinking, dunno if it'll actully work), you could make a sorta chassis for the laser. Sorta like a really thin drawer. The "drawer" will have rather strong springs on both sides to absorb impact in eith er direction. When the gun shoots back, the laser in it's "drawer" will take a little longer to move with the gun (unlike in my previous idea where the laser would move with the gun), the spring would, theoritically, absorb the sudden movement and the laser will follow the movement at a bit slower acceleration. The rear spring would slow the "drawer" down by increasing resistance as it goes farther. If the springs are right and all, the laser should "bounce" back and forth (front to back) until it stops. It's a bit iffy but I don't see how it could not work in my mind. But then, my mind isn't reality. Best give it a trial run with something else to see if it works.