i posted on c.l.f but maybe some people can use this here as well
mount a hole saw with the same diameter of the tube size you want too notch into your drill press.make a jig or mount a vise too the drill press bed and put the tube into the vise or jig at the right angle and at very low speed cut your notch.works great.
-use a wood working adjustable square too get the angle you need and transfer that too the drill press.
good luck
