This blowgun laser sight is designed for cheap keychain lasers. You'll need to provide four screws--one to activate the power button, and three to align the laser. You can customize it with whatever dimensions you want. By default, it's for a 1/2" CPVC or copper tube blowgun.
You can align using my collimator collimator: just put the laser in the sight mount, and rotate the mount in the vee-shaped holes of the collimator collimator, while shining it at something a couple of meters away. Then adjust the adjustment screws until the spot doesn't move much when rotating (up to you how precise you dial it in).