About the legendary gunpowder plot commendation

Does it have to be on a regular fort or can it be on fotd, and does the gunpowder skeleton itself count as 1, or do you need 5 skeletons around it?

Also, I wanted to ask if there was a more consistent way to guarantee chain reactions, as it seems that sometimes, even if 2 stronghold keg skeletons and RIGHT next to eachother, the other just… won’t explode