linear bearing based on ground flat stock

I'm working away on a small triaglide in the background to my other
projects to prove the principle of using Mach for the parallel
kinematics for a 3-axis machine. I now have lots of super little
bearing assemblies for the arm joints taken from hard drives and I
also have three belt drive units from Epson printers, I know these
work well and are very fast indeed.

What I need now are three linear rails and carriages, they need to be
suitable for high speed, be cheap and light. I'm wondering about
using some ground flat bar stock as a linear rail and make a carriage
that uses normal radial bearings running on this. I'd drill the bar
stock at regular intervals and support it along its length.

I've seen something like this on a few machines including the
torchmate plasma cutters. I'm interested in knowing exactly how many
bearings I should use, I could for example have a side bearing and top
and bottom bearing at each corner of the carriage but that would seem
to be overkill and I wonder about overcontraint. I can get the
bearings very cheaply so I don't mind having 12 per carriages if I
have to.

Graham