i use the laserlyte lt-pro in my 642. it works well. the cone portion does extend slightly from the end of the barrel and the on/off switch is at the very end, so it may or may not work with your holster. if you are not incorporating a draw from a holster as part of your practice, it won't matter. i am drawing from a pocket holster and when i reholster, occasionally i will accidentally turn it off.
i am using crimson trace 405 grips and the laserlyte pretty much lines up right over top of ct's aiming point. i turn off the ct when i use the laserlyte to practice point shooting.
i also have laserlyte's blue laser revolver which i use everyday; about 10 to 15 shots with both my right and left hands. it is super convenient, but it tries to approximate a j-frame. so if you are looking for a solution for something other than a j-frame, it probably isn't for you. its downside is that it is lighter, has a nicer grip, and a way lighter trigger pull than my 642. the upside is that it is a decent approximation of a j-frame and you can pick it up and "pop off" a few rounds without a lot of concern about safety. i think training with it helps me bring my ct sights onto target quicker when firing my real gun.