So after replacing the extractor spring, reinstalling the stock recoil springs, and lock titing the ejector screws, my MPX is now running like a top. However I do need mag carriers, two single or one multiple, for competition. What did everyone use back when the MPX PCC was the new hotness?

I use these:
https://www.highspeedgear.com/...extended-pistol-taco

mine came from red hill tactical. very happy with their stuff.

HSGI Tacos (as linked by creslin) or Esstac KYWIs:

https://esstac.com/sig-mpx-dou...idlength-kywi-pouch/

The Tacos use bungee tension for retention. The KYWIs use a kydex wedge insert for retention.

Both work well, it just comes down to personal preference. Personally, I prefer Esstac. (And it doesn't hurt that they're ~half the price, and even less if you qualify for their LE/mil discount.)

I guess I read more into the original request than is actually in there. What do you want to use them for? competition? home defense? range use. What kind of attachment is needed? belt/molle/?

Competition (believe that’s in the original post) on a belt.

I've got both Red Hill Tactical and the Esstacs mentioned.

Red Hill is much more competition hard kydex and the Esstacs are tactical.

Both work for their intended purpose.