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How much space do you have between the outer flap and the retaining ring on your mail slot? We'll need to determine which types of insulation might be feasible based on the allowable spacing.
Ideally, something like vinyl flap with a magnetic trim mounted on the inside may be your best bet. What I'm envisioning, I could build for less than $10 with a small scrap of vinyl from the remnant bin at my local fabric store, and some magnetic tape.
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