Sweet idea!I use a modified Allen brand rifle cartridge buttstock holder. You cut the stitching between two of the shell loops and it holds the mag wonderfully. One of these can hold 3 mags. Mine is set up for me(lefty) so it does interfere with right handed shooters unless you slide it to the other side. The way I "stack" the height of the mags makes it easy to grab them front to back.
bottom line: works great, costs ~5 bucks!