OK, thanks again for all your advice, here's where I am up to now:
The Uncle Mikes mag carrier , of course, fits fine as does the Cabela's 10 round shotgun cartridge carrier - the old 870 is always in the trunk.
The MOLLE 'pistol pouch' was WAY too slack a fit using the attached straps BUT, with the application of a very hot metal stick and introduction of a coupla' "Chicago Screws" it now fits great!.........albeit not as speedy to remove
I'm diggin' the water bottle idea and, I'm sure that this has been discussed before but, which MultiTool is generally considered the finest these days?