u/Beginning-Handle5040

Hi All, I am putting ducting in place to plumb in a new cooker hood to the outside. I am good to do most of it in round 150mm except for a short run where it will cross some surface mounted water pipes. I could do it in 2 round to oblong adaptors and about 4 inches of oblong, or I could splice in about 30cm of flexiduct and squash it to an approximation of oblong where it crosses the pipes. Is this an acceptable bodge or should I just buy the adaptors? It will all be boxed in so no cosmetic harm involved.