It's supposed to be clipped to the rear firewall. There are two small plastic clips back there, and the edge of the plastic channel just (gently, doesn't take much force) pushes into these two clips.
I think the purpose of this 'flap' is to keep the "finished interior" of the car separated from the potentially dirty / dusty / humid water-draining area of this cavity.
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2001 Boxster
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