Adding Duct/Accessory in Flex Duct


I’m trying to modify flex duct to show either a duct accessory or segment of round duct in the middle of the flex. I thought it would be an easy process of inserting a duct accessory along the curve but the flex duct doesn’t split and the duct accessory doesn’t cut into the flex.

Does anyone have any ideas on working with flex duct? I don’t mind if it’s a duct accessory or a segment of round duct used. Goal is to have it looking like below:
Spiral Duct