Paul, I'm not the expert on this, but I think that there are a few more things you need for a working system.
1. The Z axis must float over the material. Its very important the the laser beam is focused exactly at the right point. Its kind of like holding a magnifying lense over a piece of paper. You have to move it up and down to get the proper pin point of light to burn the paper. Your 5/8" rails will sag. The material will warp and bow. You have to make up the difference with the Z axis.
Maybe someone else can chime in with some real life experience. You must not be cutting much with only a 20 watt laser. Balsa wood?
2. You will need some kind of baffle to sit your parts on. Honeycomb plastic should work well.
3. A fume extraction under the table would be helpful so you don't smoke up the place.
Good luck and keep us informed
Trent