Ok, here’s A way to make a single mag pouch, in a 4-mag chest rig format. Obviously, making a single detached mag pouch would be similar but I’ll post pics of that later, too. I am using a technique very similar to pistol mag pouches, in that there is no allowance for thickness at the bottom; the material is simply folded under and sewn. When a mag is inserted, both front and back will bow together, creating a little pocket for the bottom of the mag.
The rig base and pouch panel with PALS webbing have been previously completed. We will now join the two together.
First, fold the pouch edge over and lay in place on the base. Plunge the needle. Check everything.

Stitch down the first line.

Turn the work 180 and stitch back to the start.

Turn the work again and stitch down to bottom.

Check the finshed line.

Now do the other side. Plunge needle. Check everything.

Triple stitch the line again.

Check again.

And sew on.

Sides done. Check work.

Now we’ll close the bottom seam. Line up your center lines, clamp. Form 1st fold, plunge needle into fold (not all the way in the corner). Check.

Sew down to clamp. Remove clamp. Form 2d fold.

Sew to the end (of 1st pouch). Continue to march.

And there you go.

You can view the original discussion in the forum here