The main purpose is to reduce noise and prevent algae growth and this is important as algae could eventually plug the tube.
40mm is roughly 1.5" I would be reasonably sure 1.5" corrugated tubing (as used for pool filters) or a 1.5" flex tube could work or you could carefully dremel a 1.5" fitting to fit. I would bring the box with you and see what you find. We use a tubing like the corrugated tubing as the corrugations give air a way to escape and reduce noise, it looks like what is on a typical vacuum cleaner.
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