This is just me, but if I had an already broken mirror, I'd put it flat, break it a little more (carefully by leaning on it/putting weight on it) so there is lots of spiderweb cracks and shards, then set it with something - I don't know what off the top of my head - glue/sealant that would hold all of the shards in place.
A cracked mirror almost always creeps people out due to the long-standing superstition about it being bad luck.
I'll be watching to see what the other posters come up with!