I have a hob extractor door that needs replacing, it was the original kitchen furniture style and is no longer stocked. I'm thinking of getting a piece of stainless steel sheet cut to size.

What thickness would you recommend ? 2 mm?

Also, if it was affixed to the hood frame with screws these would be visible, any ideas how to fix to the opening bracket for a better look?

Cant quite picture it treetops, is it one of them extractors that have a matching 'wooden' door that tilts out?

You could replace with a bit of wbp ply with some 1mm/1.2mm sheet attached.
A neater job would be achieved by having a small fold put in along each edge, but would cost more, probably easiest to attach a strip of wood on the inside of the SS panel with adhesive to attac the quadrant brackets to.
Either that or some small button head bolts or ss pop rivets wouldnt be that noticeable after a few days.

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Why isn't the number 11 pronounced onety one?

Thanks Hitch, yes, you pictured it right. Appreciate the advice.