I added a 20w Diode laser module to my Ender 3 - not as a addon, but to replace the 3d printing functrion entirely and just use it for it's XY axis's and control board! Check out the build video here - https://youtu.be/oWLCUFvgIuo
She's pretty easy to build! Start with a Neje 20w Laser module (or whatever module, as long as it has TTL control)
Remove the hotend bits from the printer
Open up the motherboard cover, and remove the wires for the heated bed. Replace them with a Buck converter to step the 24v down to 12v. Run the output to your laser module (If Neje, positive goes to Black and Green, negative goes to Yellow (red isnt used)
Close it back up, cable manage.
Add Z axis guide and knob if you want (3D print files here)
Plug the printer into your computer, and open up Lightburn, add a device manually and choose Marlin for the type.
Entrer 265x265 for bed size, and Front Left for the origin. Save, and then go into device settings to set the baud rate to 115,200.
Reconnect the printer, and enjoy your new laser cutter... Make sure to wear your laser glasses!!