I did this too. I wasn't sure how much weight that tip would carry so I just went overboard.
Hard to explain, but what I did was put the spool on, drill a hole through the spool AND rod. Drill where you want the cable to start winding, so it goes in correct direction of spool threads, etc. Then I fished the cable through the end of the rod (ie, the openings on the side), so the derailleur cable tip (along with the compression fitting) ends up on the INSIDE of the rod. Now you have the cable sticking out of the rod already lined up on the spool.
I can print you a couple or dig up the STL file.. it's in a thread here somewhere. I have a 2" rod also so the file should already work for yours.