Most consumer grade lasers will (example by analogy) place the CO2 tube parallel with the non-gantry axis, place a 90-degree mirror at the end to direct the beam across the gantry axis, and use a mirror mount on the gantry to divert the beam parallel to the axis, then a 90-degree mirror and focusing lens to direct the beam to the bed. Slave a spindle-on/off signal to gate the HT supply for the tube and job's a good un.

Of course, the whole lot then needs to be in a laser-safe cabinet.

EDIT: That's if you want to use a cheap 40-200w CO2 Chinese tube. If you're just looking to use a laser diode, then I don't really see much of a question to answer.