How good are you at glassblowing? [img]/ubbthreads/images/graemlins/wink.gif[/img]
The easiest gas laser to make from scratch is a nitrogen laser.
The easiest DPSSFD laser to manufacturer from existing components is a 532nM unit.
I would say you need to read sams laserfaq, however if you are seriously contemplating this you will need to absorb the laserfaq.
I will admit, if you built a laser for a school science fair you would almost be a shoe in for 1st place... Potato clocks are so boring [img]/ubbthreads/images/graemlins/wink.gif[/img]
look around for a thread called "building a laser" i dont have time to give the link now.... but that will show you what enigmahack and i have in the works at the moment