Outside smoke entering cabin (99 Civic EK)

We have a 1999 Civic EK and outside exhaust smoke (from other vehicles esp. diesel smoke) enters the cabin through the A/C system (already set at recirculate mode).

I believe the foam or whatever material that used to block the air inlet is gone.

Any EK owner(s) who can supply a D-I-Y (Do it yourself) solution ? or do I need to bring this to an aircon shop for them to put foam or something?

Thanks!