Nice work! A lot of work also! As an alternative, is it possible to use RTV and glue a foam filter sheet (cut to size) that you can get from HVAC or vacuum cleaner filters to this opening?
Or build a frame that you glue into the opening and insert a foam filter?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked