The plastic props are a lot heavier than an equivalent wood prop. The issue with the Fox .35 crank is that the torsional load of the engine firing is reacted into the rotating mass of the prop, causing fatigue failure at the discontinuity of the intake port. So keeping the propeller weight down is likely more important than keeping the spinner/nut/hub weight down.
If it was fatigue due to rotating bending loads, you would expect to see failures right where the shaft journal reduces down to the size needed for the drive hub and threaded portion--which I can't recall seeing.
The other failure that is pretty common is to see the crank disk fail between the crankpin and the shaft journal. And typically it seems the line of failure goes tangent to the shaft. Again, an area of stress concentration and maximum bending moment coming from the cyclical rod forces.
The design margins on the Fox Stunt .35 crank are pretty low all around, so you will see multiple types of failures which have different causes. I know people have had success running different props, different hubs, shaft extensions, etc. The question is how long will the engine stay in one piece? And, there are an awful lot of used engines we have collected with no real idea of how many hours are already on them. They are low cost, so if it breaks, just grab another one. Unless it fails at a bad spot and you lose an airplane that you really liked.
Here is a sampling of measured propeller weights so you can see just how large the differences are.
Dave