Coat the inside of each half with a layer of rubber cement and do your best to layer it with these as densely as possible, but only in one singular layer.
Yes. You hand them out to your fellow antifa. They light the fuse while they're holding it in anticipation of throwing it. But the fuse burns in half a second. Whoops!
With a smaller shell casing you could also put this inside it to function as a pressure vessel (even 1mm will give you vastly more boom). Still need to layer it with your pretty jewelry cubes though. You can't layer the inside so they'd have to go outside but then you need a way to disguise it as a normal shell.
Get some of these:
https://www.amazon.com/Generic-Fireworks-Plastic-Aerial-Mortar/dp/B0CZYY2X6X
Coat the inside of each half with a layer of rubber cement and do your best to layer it with these as densely as possible, but only in one singular layer.
https://www.amazon.com/UNICRAFTALE-Stainless-Bracelet-Necklace-Supplies/dp/B0D2Q1GK65
Next, install this as the fuse.
https://twistedthunder.com/shop/fuse/safety-fuse-5-sec-ft/
Seal the halves together, fill it with black powder to the brim (and AN if you have it), and then close it up with strapping tape.
Leave them in a little box, put a cute little note on it like they love to do, like "Fashy Flashers <3"
In Minecraft, of course.
Also, your suggested fuse says the duration is .5 seconds per foot. WTF?!
https://www.amazon.com/inch-HOLLOW-STEEL-threaded-weldable/dp/B07VCMF47L
With a smaller shell casing you could also put this inside it to function as a pressure vessel (even 1mm will give you vastly more boom). Still need to layer it with your pretty jewelry cubes though. You can't layer the inside so they'd have to go outside but then you need a way to disguise it as a normal shell.