How to treat Magnesium powder and ribbon
Magnesium powder and ribbon are relatively stable in air at normal conditions, probably because of thin protective layer of oxide. However, ribbon can burn in air with blinding light and powder gives self-burning (i.e. not needing external oxygen sources like air) mixtures with many common substances, some of them explosive. Moreover, burning magnesium may continue to burn in $\ce{CO_2}$ atmosphere, under quartz sand and especially under water (producing hydrogen, that can form explosive mixture). The only common material that can safely extinguish burning magnesium is iron powder. Surprising enough, being stable in cold water for short time, magnesium readily react with methanol (and probably ethanol), chlorinated hydrocarbons and hot water.

The good thing is that magnesium ribbon is not easy to lit up, and powder is almost impossible to lit up when on surface of some massive object (common gas lighter flame lit magnesium ribbon after 15-60 seconds of heating)

Magnesium must not be heated on air or under nitrogen. Period. Technical grade argon probably contain significant amount of nitrogen, so it should not be used either.

Magnesium theoretically can be melted under eutectic mixture of $\ce{NaCl}$-$\ce{KCl}$-$\ce{CaCl_2}$, but by doing so without experienced supervision you ask troubles. Still, theoretically speaking, if you find an iron (not silicate, porcelain or corundum) crucible you probably can melt magnesium in it under mentioned mixture of dry salts, let it crystallize and then carve it out.

However, magnesium alloy sheets are available on the market, so maybe you can find something and cut needed form from commercially available sheets. Try this first before doing anything requiring heating magnesium. I have doubts, that you will be able to press good tablets from magnesium powder, so if commercially available sheets and maybe some magnesium compounds (oxide, sulfide and others) won't not work for you, you probably will end with melting magnesium. I hope you won't need it, it is dangerous.