Some of the other E-flite collets may work. Unfortunately, they list many of them for the plane that they came on, and don't specify size. The one that I posted above is 5mm to 1/4 and you would have to supply a 1/4 nut and washer. If the washer is smooth, glue on coarse grit sand paper.
There is a better selection in 5mm shaft to 6mm prop collets and you can find some props with 6mm hole. I am using aluminum collets and fly from blacktop. Prop strikes in grass or crashes could bend the collet, but better than the motor shaft or bearings.