A bit of lateral thinking: "going square" is likely to be difficult (everything I've seen is rigid: eg: square section pvc drainpipes) and won't offer a much enhanced internal cross-section for air flow. Circular PVC ducting with wire reinforcement will tolerate a certain amount of abuse and squeezing-into-place and is readily available in a range of diameters and fabric strengths. Its also quite cheap. Some colourful language if the wire tears the fabric seems an essential part of the job!
For cosmetic purposes a simple square trunk could be placed over the duct to look like a cornice, but this will involve a fair bit of cutting and pinning / screwing etc.
What is above the ceiling? Any chance of surface mounting a duct against an external wall or partition above and simply dropping the extract head to sit flush in the desired location.
Probably an exercise in egg-sucking, but that's how we got around the issue. Good luck.