You could sew in another line and likely tighten it up some. You may also just trying to put some type of material in the pouch to take up some of the extra space. Poster board, card board, business cards, inner-tube material. More material might make it fit better and not fall out. Inner tube material might make it draw out slower, so just try different stuff to see what works.
I'm not sure which cargo pocket it is carried in, but a deep front pocket or side pocket in cargo pants, would generally make it difficult or near impossible, to get out with a week hand. I've posted here many times, I'm not a fan of having a mag on the holster. Here is my reasoning. I know some feel strongly about having gun and holster together and that is OK. But I'm sticking with my reasoning:
Generally, you carry an extra mag for a reload. Lets assume you have a holster with a mag built in and lets say you are right handed. You draw the gun to fire and you shoot until it is empty. Now you want to reload.
The hand you likely need to get the extra mag has an empty gun in it. So you would need to move the gun to your left hand. Then go get the 2nd mag with the newly freed up right hand. Then you need to reload the gun and likely backwards from what you practice. Meaning your gun is in the left and 2nd mag in the right. Then after you figure out the reload, the gun likely needs to get back into the right hand for shooting. You may be able to reload and shoot left handed and save a step.
Now assume you do this under stress and lots of adrenaline. Maybe you are running, being attacked, or shot at. It may not be a good time to be switching your gun back and forth to each hand. Too many ways to screw that up, drop the gun, or have it knocked away from you.
Having the extra mag on the holster essentially puts an extra mag on your body. However, as explained, it may not be that helpful or useful. I would suggest having an extra mag on the week side for access with a week side hand.