I'm not 100% on this, but I think any block which hasn't taken any damage yet will become a bag. I.e. still 600/600
I think I need to test this more thoroughly. If it is 100% chance for undamaged blocks to become lootable bags, then cutting a 15 x 15 x ?? deep frame to cause a collapse is relatively quick and easy.
Your tip about the campfires looks to be promising Might give it another shot, thanks!