Some years ago I made this adapter to mount a LaserTree  LT-40W-F23 to my Creality Cr10-v2; now my mod is highly specific, my printer is not really the most common and the laser was chosen pretty randomly(even tho I think many modules have the same mounting holes). The model is not as interesting as the fact that this mod is really easy to make yourself. 

To make such a mod it's suggested to have the ability to connect and disconnect quickly your hotend (I use a serial cable to connect the 8 wires).

The process is really simple, here  are the main steps:

1)Model up a similar model to mine or use my model if you have the same carriage and laser, I suggest using threaded inserts to get a solid connection.

2)Get a power supply that can supply the required voltage of the laser module (in my case 12V) that shares GND with your printer.

3)Step down the PWM signal to the fan of your hotend if the module has a different PWM signal voltage than your printer (in my case it's 5v). You should be fine just using a pretty high resistance voltage divider that gets you 5V and then use an opamp as a buffer at his output.

4)Connect both the power supply and PWM to the laser module, avoid powering it up if you don't have protective glasses on.

5)Install a CAD software, I use Laserweb it's free and you can easily change the code for the laser so that the software uses the fan command to control the laser intensity. 
You can find the option to choose the command for the laser under settings→Gcode, also add “G92 X0 Y0 Z0” to the “gcode start” this will make it so that the printer considers as it's origin the initial position.

6)Find some kind of honeycomb (it's decently cheap I would suggest using that) or find a piece of thick scrap material that you know won't let out off-gasses while cutting to put under your wood piece. It is useful to have fasteners that can keep it flat, that is why I would highly suggest just buying a honeycomb (I have used a piece of wood for a while with inserts, it does make for a cool showpiece and that's why I put a picture of it but it's not worth the hassle).

7)Set your printer at the right height using the measuring tool provided with the laser (you need to get the laser the right distance from the wood), also I usually set the cut starting in (0,0) and then position the tool where I want the cut to start, then just start the gcode and it should work.

!!!While the laser is installed it is imperative that you do not home the z-axis unless you make a model that does not protrude under 0 which is not really handy with how mounting points are set in my unit, my version suffers from this problem.

PLEASE READ THIS BEFORE ATTEMPTING THIS:

Lasers are quite dangerous, any laser rated to cut or engrave wood is really bad for your eyes, --make sure to ALWAYS use proper protection 

-don't go for the cheapest glasses

-make sure the glasses you get block out the spectrum of your laser

-always use glasses as long as you're using the laser

!!!many companies promote a very carefree attitude towards open-frame laser cutters I would absolutely not risk anything with my eyesight, according to laser classes any laser above 500mW is a class 4 laser (the maximum) and any laser that can engrave wood is usually above 5W (which is 10x) and any class 4 laser is considered to be dangerous even if you just stare at the dot projected on a wall (it doesn't need to hit your eye to be dangerous) 

!!!Ventilate the room where you use the laser and avoid inhaling the fumes

!!!Any tempering with your 3d printer wires can lead to damage to the printer and fire (especially if you mess up the thermistor and heating cartridge wires even tho that should not happen with modern firmwares, in fact, if it can happen you should worry about your printer even without the mod).

!!!The gunk resulting from laser fumes is quite invasive and not great for the printer, it won't be too much, it can be cleaned with isopropyl alcohol but it can build up and it can be unpleasant for the printer, I will say in quite a bit of usage the build-up was only relevant on the laser module itself which is clearly the most affected part. 

I am not liable for what you decide to do, I don't want to discourage anyone from doing this, it's really easy and cool but at the same time, it is clear that you need to be careful when replicating such a project.

My Model

If you actually end up using my model here is how to mount it:

1)Add 3 threaded inserts to the carriage holes in the main plate

2)mount the laser with m3 and m4 screws to the main plate distancing it with the spacer

3)(optional) add M4 threaded rod to the lateral holes of the main plate

4)use short m3 screws to clamp the backplate to the main plate and glue them together 

I've also included a wall mount for both laser module and measuring tool.