Originally Posted by adamsclubs
Axis, the 3" duct is air tight. The gap you see in the picture has no impact.
I had tried a 4" ABS coupler (actually a 3" coupler with 3.5" ID), could not fit in the tight space.
Apex's mod is the best you can get with the stock box.
I am referring to how two of the plastic 3" adapters had to be used in order to make an air tight seal. There is a slight gap around the square base of the adapter that makes using two a necessity. I am hoping the 4" one will cover it completely. Check the picture. The red box is where i'm thinking the 4" adapter I bought should cover the gap. You can see the gap I am referring to with the green arrow. And the filters I am looking at with the 4" fitament are available in the same range of width / length as the 3" inlet one, so there shouldn't be any problems there either.