Ian White GM3SEK wrote in
:

Forget the scribed line - what you really need is for all the holes
through the boom to be parallel. A spirit level clamped at 90deg to the
boom will achieve that.

I have tried that, and it works fine... so long as the drill stand is
fixed.

The other thing that I use that works well is a rod which I clamp to the
round stock (it is actually part of a device for hanging camping lamps
from a tent pole) and I visually line the rod up with the drill while
adjusting the angle and tightening the vice in a drill stand. The
advantage of this is that I can see the rod at the end of even 6 metres
of boom from the drill, whereas I cant see a level at great distance and
need to walk back and forwards making small adjustments.

Another variation is to drill the first hole exactly the size of some rod
(I use some 1/8" brass or stainless which I usually have on hand), fit a
short length of rod and tape it in place, then proceed as above. Then, if
necessary, drill the first hole out to the final size.

We don't usually drill the holes for an interference fit, but rather a
small clearance, so you should expect to need to align the elements after
assembly.

Owen