Return Air Grilles Reece

How would you go about adding a filter slot to this? : rhvacadvice return air box with filter slot air out, but ALWAYS pulling air back TOWARDS it Your filter slot is almost always on this return of kong slot Modules · 5 Gas mask air filter · 25,000 Dollars · 5 Military power filter · 3 Military corrugated tube · Skier LL3 · Level 3 Generator · Level 3 Vents

return of kong megaways slot I was changing my HVAC filters yesterday and I noticed in the air return box slot inside the 20x20 intake box I slid the filter out