I posted this in my pack build awhile ago and it seemed to be popular, so I wanted to make it easier to find. If you're building a plastic N-filter, I link to the tubing I used for mine on that thread.
Print the template at 1:1 scale (don't fit-to-page) and cut along the outside. I've left some overlap in the template so that you can tape one end to your tubing. Wrap the template around the outside, and then tape the other end down. Make sure you drill small pilot holes in the center of each circle first. Then go back and drill your 1/2" holes.
There are two templates, depending on the size of your N-Filter. If there is another common size, let me know and I will add a template for it.
N-Filter Drill Templates v 1.0:
3" Outer Diameter tube template [download]
2.75" Outer Diameter tube template [download]
Hope these help!