Use a plastic shot wad or paper shot cup, then test. If unsuccessful then get rid of the Mag Spark, then test. If unsuccessful then switch powders, then test some more.

IMHO nothing will matter much except a plastic wad or paper shot cup.
Protect the base from flames with the TC wool wads.
2 wraps of paper can be used for the paper shot cup.
If the paper cup doesn't break open before it hits the target, then make the slits bigger on the sides of the paper cup, or add another slit.