All right, then. Thanks for all the help! I think I'll measure the side of the 870, and make a template with cut-outs for the Trigger group pins and the serial number, so they are observable and serviceable, and then apply the industrial Velcro once I buy one each of the max practical size from 3 Gun Gear and Esstac, and then evaluate.
As mentioned, I have plenty of appropriate size pouches, as well as loads of SG shell carriers of all types. I would think that a half-dozen or so cards, with one on the SG would be sufficient (perhaps more than sufficient), as I can put un-carded SG rds into the other carriers, if desired.
Any further comments and advice will, of course be more than welcome, so feel free to chime in.