Firebox cover in attic

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Hey everyone im looking for some advise.

In my recently bought home I can look down from the attic and see the top of the firebox. The hole is about 4 foot square and about 7 feet down and loose fill insulation has fallen down on top of it and around it. There is also about a foot of space on the three sides not up against the brick. The flu pipe isnt touching the sides but after comming up strait goes at an angle up to the roof.

I'm planning on going down there to clean off the insulation that I can get but my question is should I have a cover over it in the attic? I was thinking of just making a dam out of flashing so no more insulation can fall down or should I cover it completly? If I covered it I think plywood would be ok since it would be so far from the firebox and just use metal around the pipe. Should the cover be insulated so im not loosing heat through the metal walls of the firebox?

The house I bought is not exactly quality and I think the contractor just didnt care to finish everything.

joecaption

A picture may help clear it up for us.