Product review (3 stars): Having this filter for my humidifier is a 'must have' ...
Having this filter for my humidifier is a 'must have' apparently.  Unfortunately it comes in a flat box so it's more or less folded, and is tough to get into a circular track at first.  After it's wet I'll wager it becomes less rigid, the folds and the straight edges likely morph into a circular shape, and it probably then self-adjusts into the track, but I do find it a bit of a pain when first installing.