Its really hard to do, becasuse its a lot of func_trains to have the beams start and end from in an enclosed space. read the train tut and the env_beam tut. and look in the example map section too.
and if you ever decide to build a full laser box, my only advice is to make as many copies of the room as you have lasers, plan out one laser path in each room, then when your all done delete the copies and move all the setups into one room