Bikejunky, you probably could kludge something together but I doubt that you would be happy because the direction in which you're trying to go isn't likely to be profitable. Well within the first few hours of a good, intensive combat shotgun course you'll probably be cursing that cheekpad/shotshell holder and want it off.
That shotgun is fighting itself and you'll probably be fighting it. One of my Remington 870s is the rough equivalent of yours: 18" barrel, rifle sights, 2 round (not 3) magazine extension, and SpeedFeed stock. The forend on mine is a SureFire weaponlight, and it's there for a reason not as adornment or to impress an attacker with my tactical coolth. I do have a SideSaddle for it but I've never figured out whether or not I need or like it, so sometimes it's on and sometimes it's off. For the past few months it's been off.
My own experience with this kind of shotgun is that it needs to be highly maneuverable and as lightweight as possible. I regret the weight of the SureFire forend but, as I said, the light is essential for me. The SideSaddle isn't essential, only nice to have six extra rounds onboard, so because it makes the shotgun klutzier I and adds weight can't decide finally whether it adds or detracts. You know my current decision. Next month it might change. Sometime after that it likely will change again. I wouldn't add any more weight to mine under any circumstances.
A shotgun for home defense is artillery in Massad Ayoob's view, and I agree with him about holing up in a safe room and holding off home invaders while awaiting the arrival of the police. Clearing rooms is not part of my job description and never will be if I have anything to say about it. That said, I still want that shotgun to be light, fast, and maneuverable because I can't count on anbody asking me my preferences while trying to kill me.
Instead of beating that moribund horse any further, let me make some positive suggestions.
Consider losing that cheek pad/shell holder. With the magazine extension and SideSaddle you're already hauling around thirteen or fourteen rounds. If you have the shotgun slung around your neck and need to drop it for the purpose of transitioning to a handgun right off the bat you'll be dragging around a boat anchor. Imagine what you need to support when you add another five or six rounds in the cheek pad/shell holder. That's not only added weight but also lessened maneuverability.
We might be built differently but I have rifle sights on several different shotguns and don't need a cheek pad to use them. My cheek just goes where it's supposed to go on the stock and welds there.
Perhaps adding to the puzzle is that my shotguns all are cut down to 13" length of pull instead of the standard 14". That lowers my eye. The cheek pad raises yours. I wonder if I could mount your shotgun fast and hit anything unless I moved my cheek to some awkward position for me.
At any rate, without that cheek pad/shell holder the world of slings opens wide. I've used simple carry straps built according to the specifications someone else gave you here: those are only a way to carry the shotgun safely at the range. For other purposes I've worked with various other slings. Most aren't agreeable to me. Two stand out.
The two I'd recommend to your consideration are the inexpensive
Boonie Packer and the much more expensive
Vickers Combat, which is available also in a
less expensive Victory version. Take a look at their web sites, see which interests you, then call them with a description of your shotgun and ask for their advice about what will work best for your needs. I've found them both to be trustworthy advisors.
Another possibility is perhaps less realistic for you because it requires some small modifications of the shotgun to accept the H&K GP-3 web sling that is now hard to get. And its best use invoves a learning curve. The other two I mentioned mimic some of those uses, are easier to acquire and install, and probably are quicker to learn.
Whew. I'm ought of breath from trying to pack so much into a relatively short space. I hope it makes sense to you.
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Flip180 posted just before I did and I hadn't seen his message until after I posted this one.
Look at his shotgun to see an illustration of what I mean about light and maneuverable. Look also at his dump pouch, and do get one--a good one--for yourself. Hang it on your belt at about 7 o'clock and dump two or three boxes of shells into it fast whenever you get a chance. That's your ammunition supply. Just make
sure not to mix slugs, buck, and birdshot in it. Never. No. Nyet. My favorite dump pouch is the one from Maxpedition because it folds up unobtrusively when not in use, so I tend to keep one on my belt most times when I'm near firearms. It's a habit, nobody ever seems to notice it, and the one or two people who have noticed thought it was a small case for Kleenex. (People perceive what they expect.)
