how to cut tub stock

[hal feinstein]

Here another simple but elusive construction questions:

I built a simple 2m ground plane antenna.  The ARRL handbook uses a  
pipe with
a 3-4 inch cut line down the center of the tube stock, one cut on  
either side of the tube
so a pipe clamp can be placed over that end of the tube to tighten  
it.   Question:
how do you make these cuts, one on either side, neatly.   Unaided  
(with hacksaw)
you'll find it very difficult to keep the saw on track.  It wanders  
and you have to correct the direction
your cutting which looks like hell.   A dremel tool was  
suggested.      So, how do the "pros" make this
kind of cut?