https://www.triadtactical.com/FTW-rear-bag.html https://www.triadtactical.com/Red-Tac-Gear-Rear-Bag.html
Both are very good bags. I have abused the Red Tac for years and it has served me well (very well). I prefer it with the 308ar. Both can be thrown on a barricade, tucked under your arm for standing sling shots, put in a hog saddle, placed under a knee......I mean they are durable. Throw a grimlock on them and they will be with you. The red tac can make some noise when you are running.
I have an old piece of an army duffle bag filled with beans that was my go to for the bolt gun. I have worked with guys who did the home brew bags....some worked well, some didn't.