Thanks for the help. i will be trying this out soon.its called a street elbow. one side is 3 inches. the fatter side that would take another pipe inside it is 4 inches on the out side. cone filter fits right on it. if you are going to a pipe then you need a filter that will fit a 3 inch pipe. spectra filters fit a 4 inch, but have a ring to reduce it to 3 inches.
the roof flashing you'll want a 3 inch boot to fit the pvc pipe.
will 10.5 work? i found one on amazon for 40bucks. its a eldelbrock cone filter4 inches in diameter and 7 inches long
it fits nicely with a lot of room so you could go bigger if you like