Not what you asked but...
FWIW, most plate carriers don't work well without plates. If you're going to use plates, then obviously you can ignore that. If you're not you'll be better served with a chest rig. I really like Eagle for this, and they sell a Molle Chest Rig
that works well for this.
Regardless of which of the above you choose to use as the platform, the Eagle M4FB pouches
are the best of both worlds and offer the ability to run them open-top or closed. The kydex lining allows you to run the pouches open-top for 99% of usage but they have a secure-able flap if you find yourself needing better retention.
What I use is somewhat of a combination of the two. The Eagle M4 Chest Rig
combines the kydex retention of the open-top magazine pouches but they are sewn-in to the chest rig. The pouches don't have the flaps, but I've been using one of these for a couple of years in multiple classes and matches without issue.