Have fun, BE SAFE! Fireworks ought to be built and tried by those with experience in the matter…. anyway, you knew that already.

Quite a simple snap-fit sphere- any holes needed can be placed into it by means of a negative volume in Prusaslicer or similar. This was a simple job in fusion 360 to figure out how to make something that fit together like an easter egg might, I thought it interesting. You can create your own variants in fusion with circles, offset planes, and the loft tool. It works nicely.

Of course, you will need to glue this together once you have packed it with your stars and break powder. The shaping of the inner counterbore is such that it bursts along 40 degree meridians, or simply, through the top and bottom, but with a little top hat to go with it. 
It can vary with infill- it could make even a rudimentary break charge sound nice and loud, or smother a weak composition and make it fizzle out like a flare. 
Also due to the nature of the countersunk ends, drilling out the exact center for fusing should be a piece of cake!
yes…. you can scale this up to some interesting proportions…!
Print any orientation that you want to, be aware the fit between the two halves is .11mm at current size.