Where to start fixing a coal shute/chute cover?

[hugoagogo]

Victorian terrace complete with coal hole. Currently it has a sheet of tooled metal just plonked on top-not terribly secure.

I have tried googling and can only find round ones (ours is square) and v expensive.

I'm not that bothered if it looks properly victorian, just so it's safe. How can I fix this piece of metal to the bricks? Or can I buy something that can just be slotted in?

Also, should I be trying to make it water tight, or should I keep it ventilated?

No idea where to start.

Thanks in advance