I've done this a couple of times. My thread on the Tank007 E10 mod (https://budgetlightforum.com/t/-/22831) ,I used a SinkPAD, but you can do it with a Noctigon. One trick is to file the edges at the pads at an angle so the pad edge is no where near the housing it's sitting in - worked great for me. I did a similar mod to the SingFire little SF 348, pics here: http://s1054.photobucket.com/user/TomE2012/library/SF-348.
Always check for shorts of course. I used a rotary tool with the sanding drum I believe. I don't have any trouble getting things round for some reason. Mark your target with a marker - O-L style, and use that as a guide to how much material you are taking off.
The SinkPAD's were made for this with the two pad set, but Noctigons will work too, specially for 13 mm.
Also post #422 here: https://budgetlightforum.com/t/-/27548 for the SF-348.