If you're willing to spend $75, have you considered Wilde Custom or Mystery Ranch? I own neither,, my stock packs are cheap amazon ones, which have been surprisingly durable and useful.
The Wilde Custom Gear Rifle Stock Pack provides storage on your rifle stock and allows instant access to extra ammunition with removable ammo cards.
wildecustomgear.com
Tactical Nylon Mini Rifle Stock Pack was designed as a solution for semi-auto rifles, such as the AR15, that utilize adjustable stocks with a smaller profile like the popular Magpul MOE stock.
wildecustomgear.com
Kill two birds with one stone with the CHEEKY RISER. This rifle stock pack offers key storage space for ammunition, Ballistic Data Cards, or tools, while providing countless riser adjustments.
www.mysteryranch.com