Grind that magnesium block into shavings or powder and burn it, instead.
Tried that too.
I filled a cardboard box, about 6" cube, with magnesium cuttings from a turret mill. Actually I emptied the contents of a hoover which had been used to hoover up the swarf during milling. I then set fire to the box with a blowlamp, stepping back fairly quickly....
What happened?
Not a lot. The stuff burned very slowly.
The problem was obviously a lack of oxygen. To make magnesium particles go up fast, when packed tightly enough to ignite from each other, one needs to mix the stuff with an oxidiser of some sort.