How about using one of these....
https://www.screwfix.com/p/mcalpine-mac-exta-pan-connector-straight-extension-piece-white-110mm/98788
Then running a bead of silicone round it (https://www.screwfix.com/p/no-nonsense-builders-silicone-clear-310ml/83710) once it’s in position, screeding round it...