First of all I've just finish my 16-43 Shiva Sonotub and it look great(sorry, i got no Digital camera!) and sound very deep! Thanks for all the help I've received in the last 6 month from you guys, and especially from the people on the Sonotub project link where I've taken all my ideas. And a specials thanks to Patrick Sun's Great DIY Sonotub guides... What a great help it was! now, I've got a 4" port on the top of my sonotub. So I've decided to place a small 5" speaker grid on top of it. but after testing this week, I've discovered that on frequency below 25Hz the air flow on the port cause some "chack,chack,chack" sound, when the air hit the grid. So I've decided to remove the grid and problem solved. But I don't really like the idea of having mu sonosub with a big 4" hole on top and nothing to protect object from falling in! So I am thinking of adding a top plate similar to BrianW's Flexi Sonosub . with 3 1/2 " leg and a MDF top plate. But I need to know if putting a plate 3 1/2 " from the port hole will cause degradation of airflow in my port and therefore reduce the output of my sub? Just want to know if anyone experienced any problems with similar design before I've start doing it?