A small heater core should almost fit directly into your intake's filter box (provided you use the original filter element). It also won't restrict enough airflow to make a difference, unless you drive at somewhat high RPM's on a regular basis... however, with the "fresh air" flap, you could easily get colder air and better flow for "performance" driving, as it were.
Just an idea though, and not fully thought out, by any means.
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