Take a piece of 4" Sewer and Drain PVC pipe, perhaps 6" long. Use a
thin hand saw to cut slots about 1" long at each end of the pipe,
perpendicular to the length of the pipe. Space the slots equally so
there are 8 slots on each end. You can now use a 4 1/2" band clamp on
each end to clamp it to the port and blast gate respectively.
I use a Japanese razor crosscut saw to cut the slots. Be sure you use
the flat stainless band clamp, not the crappy wire type clamp some wood
supplies sell. Lowe's and Home depot sell the flat band clamps.
Don't use any duct tape or other type of tape. It will creep and slip
over time.
Bob