I decided to make my own from a radial arm saw and used the carriage and motor attached. It took about 2 months to make the frame, cut the steel angles, make the spacers, counterbalance the weight of the carriage and make a blade shroud to collect the dust. I had no plans, just solved each issue as it came up. I originally was going to have it lay flat, but it took up SO MUCH floor space, that proved unworkable. I tilted it up at a good angle for resting the panels and went on from there.
There are many ways to make sliding mechanisms that have little or no friction, ball bearings like the Holz-Her uses, V groove bearings that ride the edges of angles, and the low friction 80-20 bearings available on EBay.
There is a kit from Saw Trax that has the bearing plate and tubes you can get to make a vertical saw, but it won't travel horizontally unless you add more bearings and a sliding carriage arm.
http://www.sawtrax.com/Panel Saw/panel_saw_kit.html
This is a pretty good workout for the horizontal and vertical travel:
Vertical panel saw - YouTube