You might consider thickening the web for your plastic ones, which only requires adding a glued-on filler disk of plastic or metal to the pattern you have just made. (Preferably make it very thick at the center near the hub, and quite thin at the outside so you still have almost the whole face width of the internal gear available.) However I think a plastic hub most likely won't stand up: I think the key will mash the plastic, under load. The simplest solution I've thought of so far, is to machine or grind four large flats on the outside of the original steel hub, so it is just about square. That should reduce the stress on the plastic to around a fifth of what it would be with a plastic hub. If you eventually take up making these gears for other people whose plastic gears have broken, you could require them to supply the old hubs, with their orders. All you'd need then is a simple fixture to hold the hub while you create the flats.
