There a plenty of online videos about HVAC duct work that show installing a "start collar" to the side of a duct that is wider than the collar. (e.g.https://www.youtube.com/watch?v=ZUlRYer9-3Y ) Are there any that show the case where the duct is exactly the same dimension as the collar? (e.g. 8 inch start collar to the 8 inch side of an 8x24 rectangular duct) Or is there a special fitting that is used in such cases - something different than a start collar?