X axis load cantilever?

I've just spent a few hours with pencil and paper, doodling out some
layouts for my (theoretical at this point) PCB mill/drill. I want
to have roughly 18" x 18" usable area, for the Y and Z its not a
problem (gantry style) but for the X axis I figure I'll have to get
MUCH longer leadscrews and rails. But how much longer?

If I fully support the 18x18 X-axis table with say, 6 linear
bearings, one in each corner and one in the middle, I'll need a X-
axis length of roughly 36" + bearingwidthsetc in order to access the
entire PCB. I can shorten that by pulling the corner bearings
towards the center and letting the ends of the table hang in the
air. If I used something like 1/2" aluminum for the table, how much
overhang do you think I might be able to get away with? Keep in mind
I'll just be doing light work on this.

I also considered a stationary table and moving the gantry - but
that looks a bit scary... Have to use two matched screws and timing
belts etc.