I played around with making powder from scratch as a lad back in the 60’s.
My interests were not malevolent, just scientific curiosity.
I was just trying to make my own homemade fireworks.
Never used anything any more sophisticated than a fuse to set them off.
I still carry burn scars on my hand from those days!
Seriously, public trashcans seem to be an obvious place to put a bomb. How many bomber movies had the bomb in a public trashcan? Maybe frequent checks of these cans at public events are necessary. Given that pedestrian trash is rarely large & bulky, trashcan openings can be much smaller. The cans can be placed in the open so that they are easily observed by all. They should be emptied just prior to a public event. If these bombs were on timers, then they could have been there for hours.