Hey Brother instead of the cardboard try using the reverse side of velcro "the soft side" ..it's sticky and it will remain in place unlike the cardboard. I just cut 2. 1.5 inch strips and inserted it on the two outside at the backside of the mag holder, ones that you were talking about and it works awesome. been doing quick magazine changes during target practice and it's working flawlessly. you can even reinsert them easily if necessary.
How about using a heat gun & pushing in on both outside sides with a piece of round stock to bend in & take up a few mm of internal diameter? That should make a tighter fit.
Just a thought, but I'm wondering if the unit could withstand a simple relief cut down the front of the middle holder to relieve the excessive tension there on that mag? It's a good system and I am considering runningbthis relief cut test on a sacrificial unit.
so the double mag pouch would not have the same problem you figure then? Also you could try using some superglue to keep something in against the back on the two outside mags so on a range day you dont have to keep picking up your pieces of paper