Everyone has different situations and needs. A scabbard is a great idea when on the go and and you know you're gonna use it. Lugging my 500 around by itself, even with a shoulder strap, is clumsy. I want my gear tight in rough terrain. I use one with two straps worn like a (backpack) if I'm out clearing rattlesnakes. Or I fix it to my larger pack when wearing one. I go with a full stock on my persuader if I know I'm gonna use it, for obvious reasons. When I take my 500 "just in case", meaning I don't foresee using it, but want it anyway, I do swap out to the pistol grip. No accuracy at range, but I can sure as hell hit a brown bear at 20ft when it's chewing up one of my dogs. Certainly, as one of the naysayers mentioned, a large caliber handgun is better choice in most situations. If you got the money to drop, go for it.
A JIC-like survival kit has it's place though. I think it goes without saying you'd want to swap out whatever trinkets they give you for real gear. Waterproofing does have value for some of us. On a few occasions I've found myself swimming with all my gear (unintentionally). And around here we swim in snow 6 months of the year. I'd love a stainless mossy if someone else is paying for it. Mossy's tube is a nice design, but they won't sell it on it's own. And I doubt my 20in barrel will fit in there anyway. The tube is a bit bulky and could be smaller in diameter if something was done to accommodate the grip. If it's really just along for the ride in your trunk or boat, then PVC with some silicone isn't a bad idea. You could rig it with a wire under the silicone that could be pulled to break the seal... like opening a pack of cigarettes. Personally, I want something that allows for easier access on a more frequent basis. I have seen fishing rod tubes with enough diameter that could be shortened. I'm going to try and put something together. Might try to accommodate the stock packed separately as well. Will post the results if anyone is interested...