I have an old Derco Grizzly catalytic stove that has a top exhaust flue collar that has an outside diameter of 6.5 inches and an inside dia. of 6 inches.7 Inch is WAY to big and 6 too small to fit correctly on the collar. What's the trick? Can anything be done with the average sizes (6,7,8) of connections or is this some obsure special order item. Teach me.