As my little foray into magazine holding stocks failed miserably, I changed the problem I was trying to solve - easy access to ammo.
Have just received a nice Condor recon rig, with a VT holster and hydration pack that clips into the chest rig.
The chest rig holds 6 M4 mags and 6 pistol mags, so that should be plenty. And the hydration pack will fit a can of gas, spare BBs, speed-loader etc.
Have had a play with the holster and it'll fit both my 18c and mk23, which is cool. It won't take the mk23 with its suppressor though - however, I'll only be using it outdoors, so can't see that being too much of a problem.
Have also received my angry gun sling plate. However, annoyingly, the castle nut wrench has not arrived, so I can't put it on for tomorrow.
The only sling mount point I have is on the end of the stock, so I'll have to be careful!
*Edit* - and the Condor stuff is all going back. Well made and sturdy, but feels very unbalanced and hurts my back a great deal fully loaded. Back to the drawing board.