With hollow rifle stocks or pistol grips, I store gun-specific support items. For instance, in my AR pistol's BCM grip, I keep a couple of spare CR2 batteries for my optic. I wrap those in zip-top sandwich bags to prevent them from contacting eachother, and also to keep them from getting wet. Also, a tightly wrapped sandwich bag will prevent any rattling. This is helpful for operators operating operationally, or for hunting purposes.
My 20" AR wears a scope, and that merits carrying a lens pen. Rather than let one float around in my range bag, or a rucksack (if I were still an operational operator), I keep it wrapped tightly in a plastic bag in the pistol grip.
As for the capacious Magpul stock, I keep a reasonable field cleaning kit. I stripped down an issued OTIS cleaning kit. I have the cable, .22 wire brush, .22 patch loop, pull handle, and infamous green toothbrush. If you have room, a small non-aerosol lubricant source would be great. I also throw a few q-tips in there in case the bolt were to need deep cleaning. All of this goes into a zip-top bag for the aforementioned reasons, plus if you have lube in there, you'll want it wrapped up because it will explode and coat everything in CLP. CLP stinks, and if this is your coyote gun, ol' Wile E. Coyote will smell you a mile away.
So what doesn't go in these two spaces?
1. Medical supplies.
I have a blowout kit in my range bag. It's full of leftovers from the Army, but it has the bare essentials: exam gloves, two CAT tourniquets, one medium bandage, one small pack of surgical gauze, one medium QuickClot bandage, and some medical tape. Check your expiration dates, and NEVER use QuickClot for anything but very serious wounds. If you aren't trained in the use of it, don't carry it. Tampons will do nicely, and are FAR cheaper. A blowout kit does not include: ACE bandages, band-aids, Neosporin, or Motrin. That would be a very basic comfort care kit, and sorely insufficient for emergency medical needs.
Also, for the operational operator, if you get shot ( in combat, your med kit is for YOU, not THEM), your buddy will have to take your rifle apart to treat you. This is unhandy if you are still in fighting shape, and wastes time vs. just having your IFAK on your vest.
There is something to the "last mag" theory where you keep your very last bit of ammunition somewhere separate. Inside your rifle's furniture is the wrong place because you can only store loose ammo there, and that is dumb on a bunch of different levels. Instead, keep one magazine in your assault pack, or on the back of your armor. Or if you're just competing in 3-Gun or coyote hunting, don't worry about it at all.
Are you Dugan Ashley? No? Lose the skittles.