I make my own holsters and mag pouches.
For a mag pouch, draw an outline of the magazine. Allow about 3/8" margin all around -- perhaps a bit more on top. Now cut that outline out of just about any old leather you have -- it doesn't need to be saddle-skirt leather. (Call this the First Piece.)
Make a second piece, this one about half the length of the magazine. (Call this the Second Piece.)
Now cut two straps -- not so wide as the magazine, and about twice as long as your belt is wide. Use an ordinary snap kit (available at any sewing supply section in any large store) to put a male snap on the end of one strap, and the female on the other.
Snap the pieces together -- with the ends without the snaps even. Put a little contact cement on the first 1/8" of the ends without the snaps and let it dry, then press the pieces together. The straps should now be fastened together at both ends -- one end with the snaps, the other with contact cement.
Then put contact cement on the outside of the top edge of the Second Piece, and some on the top of the two straps (the same place you put it before, but on the opposite surface). Press together.
Now stitch the parts together -- I like to mark the stitches, drill through with a tiny drill in a drill press, then sew with waxed thread. Double sew -- complete the sewing, then go back over it -- going in to holes you went out of before.
You now have three pieces of leather, with the straps hanging down and snapped together.
Next, use contact cement to draw a border about 1/8" wide around the First Piece from the bottom (and including the bottom) to about as high as the Second Piece. Make another border around all but the top of the Second Piece. Let them dry, then stitch together as before.
Finally, take a magazine, wrap it in Saran Wrap and weat the leather with alcohol. Force the magazine in, and make the leather conform to it. When the leather is try, use some Sno-seal to make the inside slippery.
To use, stick the carrier and mag inside your waistband, slip one of the straps under your belt, the other one over, and snap. Voila! An IWB mag carrier!
