well, most people use aluminium plates 10mm to half inch thick, for structural rigidity and good thermal dissipation (although others would argue and not be unjustified, that you should not dissipate too much heat through the baseplate, as this upsets optics alignment over time)
a nice guide to near field and far field alignment can be found here (you will need both kinds of alignment) http://www.stanwax.plus.com/lw/rgb%20alignment%20lw.pdf
something i strongly recommend is the implementation of "bounce mirrors". A nice example can be seen here http://www.photonlexicon.com/forums/...4&d=1308164963
The beams bounce off a mirror before heading to the dichro. this extra adjustability can compensate for drilling, milling, building discrepancies and beam height differences (laser to laser, laser to galvo etc etc)
other that that, as taggalucci said, there is much much more available around here, use tha search and be sure to come back with build pics. everybody enjoys these, it is like our reward
"its called character briggs..."