This is the bag I use...
https://www.accu-riser.com/e-700-eastern-possibles-bag/The bottom of the flap is pretty close to even with the bottom of the bag. Underneath the flap is a small pocket with a button closure for anything that I think might bounce out while moving... Which I have never had an issue with anyway.
The bag I carry when I'm out with my .50 has a flap with a small hole that a thong pulls through. The thong gives a small measure of extra friction to keep the flap closed. And I can tie a simple knot that pulls loose with one hand for more security.
But again, I have never had an issue with stuff bouncing out. Of course I don't make a habit of running with my gun and gear, and have never had need to hang upside down while carrying the bag.
I agree with each gun having its own bag assigned to it. It keeps you from having to load/unload the bag when you switch from gun to gun and need to pack specific gear for that day's chosen gun.
The gun may also help determine the bag you choose... A rifle doesn't need much to keep it going. So a bag for a rifle can be pretty simple... Perhaps a belt bag or just stuffing your pockets with what you need. A smoothie has more options for shooting and may require a bit more bag to keep everything organized for switching from ball to shot, patches, wads, etc...
The bag I linked above is really too much bag for my .32 I usually have to resist packing it with more than I need. But I have it, and it i ever decide to get a smoothbore i think it will work well and I can just buy (or make) a smaller bag for the .32
I probably just gave you more stuff to think about instead of helping you narrow down your options.
MIke