You want the hub to be lighter than the blades (since a gyroscope functions best when the bulk of its mass is on the outermost edges). It’s best to narrow the blades as they approach the hub or cut a hole in the hub. This also prevents premature layover.

Parabolic wing tips are better than flat.

Different materials have different optimal lengths and widths, so experiment. In general, heavier materials mean narrower wings.

Experiment with heavier, more rigid materials, such as the corrugated plastic used for political-campaign lawn signs. Sign-making shops often have scraps of interesting, dense plastics that are easy to work with hand tools. The throwing and tuning tips still apply; heed the warnings in Store-Bought Boomerangs in Store-Bought Boomerangs when throwing these!