What mag holders/pouch fit the magazines
Generic double stack mag pouches fit if you can adjust the width, I use Unckle Mike's. CR Speed STI mag inserts work with a 0.055" washer behind them. CR Speed has a prototype W/B spacer that is the correct fit in both directions, but I found the much larger STI insert with the washer to be functionally easier to use.
Here is my feedback to CR Speed:
While the W/B spacers do indeed fit the Alien mags (The thickness of .280” is correct and matches 0.225” STI + 0.055” single stack washer), we found that the STI spacers plus the washer were a little easier to insert the mag, given that the STI inserts were physically larger and the ramp length on the STI spacer was longer. The fact that the W/B constrains the mag in the other direction was also detectable as a slight increase in effort. You could easily tell you were inserting the mag until you hit the W/B spacer, and then pressing again to overcome the (second, perpendicular?) spacer lip, while inserting the mag with the STI spacer was a smooth motion. Granted, given that the STI insert only shims in one direction, there was slightly more movement in the other direction with the STI insert when the mag was seated. It is also understood that mags are not generally inserted under a time constraint, so a slight wiggle or extra effort with an physically smaller insert or one that constrains in 2 directions is not a huge issue.
If we were designing an Alien insert, it would be a .280” thick STI insert or would be the W/B insert, but occupy more of the physical depth of the pouch, (so deeper when viewed from above) instead of just the middle. Or we would supply a 0.055” flat shim with a hole in the middle for the 0.225” STI insert to fit the shim to use with the Alien. We will likely fabricate this 0.055” shim here.