I'm carrying shells directly on my Wilderness Instructor belt as follows (bear in mind I am right-handed);
On left side, between the 9:00 o'clock and 11:00 o'clock belt loops, four of these which are used to hold eight rounds of Federal LE132 00 buck (flight control, reduced recoil);
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
They ride on the belt very securely and position the shells nicely to use my left hand to keep the magazine topped off. The belt loops keep them from sliding fore or aft.
One the right side, between the 1:00 o'clock and 3:00 o'clock belt loops one of these to carry six rounds of slugs;
https://www.thewilderness.com/magazi...er12-12-gauge/
In the two end loops (which are offset from the center four to easily distinguish by feel) I carry a couple of the nasty-ass, engine-block-busting Brenneke Heavy Field Short Magnum 1-1/4 ounce slugs. The center four loops carry Federal LE127 RS reduced recoil 1-ounce Hydra-Shok hollow point slugs.
I carry a PPS or PPQ in a JMCK #3 IWB at 3:30, just behind the 3:00 o'clock belt loop and the ZipLoader12 does not interfere with it.
Finally, I carry four rounds of the Federal LE132 00 buck directly on the shotgun. If I pre-load the magazine with five rounds of 00 (mag holds six but I leave one slot open if I need to go slug), I can have 17 rounds of 00 and 6 rounds of slugs on tap. What I like about this arrangement is that all shells are within easy reach and quickly accessible, and most importantly, are held consistently in the same places.
And of course, it's nice to have options.