I tackled the Matrix takedown spring area this way - cut a second spring and stacked them. i should have used a small washer to separate the two springs, but can't find a washer small enough. I doubt the springs will connect (spiral together at the ends) based in the amount of movement there is there. If they do, it's not a big deal, not a high-risk area.
I figured I needed the spring to stick out as much as it does on any other AR15 build, which is about this much:
I stacked the two springs together (detent and takedown pin in place), and just cut it off there. Looked right.
Length on both springs, end-to-end:
Didn't feel like drilling the lower to use a 6-32 set screw, so I just ran the 4-40 tap in the stock hole - no drilling required to run a 4-40, if you decide that.
4-40 set screw going in:
Done: