here is a simple design for a home made mirror mount
http://www.eio.com/repairfaq/sam/mmount1.gif
Im sure there were others on the laser faq, but i cant seem to find them :S
In practice, unless you can find screws with a very fine thread, and tap threads to suit, you wont get a great result. We home made some early on, but the results obtained weren't that good, so we gave up and now don't quibble over the cost of real mounts
Perhaps what you could do Andy, is do it on the cheep, then when you make some cash from laser shows, reinvest it into stuff like pro mounts