The so call flower type lens hood are design to fit specific lenses only. Even though a Zoom lens don't have rotating front element during focusing, or neither do it extends when zooming, this does not means the focusing mechanism is the same internally. Different lenses focus differently. It would be pretty risky to assume that and flower hood designed for example, 28-70mm Sigma, will fit nicely on your own Canon 28-70mm because both have the same focal range, non extending barrel. The lmage size will change during focusing and zooming, and these hoods are designed based on the characteristic of the lenses intended.
If you want a Flower hood, get the original. If your lens doesn't have it, then get another lenses that has one made specially for it :bsmilie: