Minuteman International (Black & 32 x 40) - DGC-01 40 x 32 Draught Guard Cover

Is this safe to use with a dying fire? I have to keep flu open due to embers. Or does it get too hot?

How do I use the magnets? I have a fireplace with a brick exterior.

It says it's 32 x 40 but my fireplace is standard and it measures 31 x 39 . I need to know the true measurement.

Does this item stand up on its own or do you have to use the magnets?