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Attaching a nylon gear to the end of a shaft

I’m struggling with attaching a nylon gear to the end of a steel shaft. It’s part of the drive for a small cart that is hand pushed with a 5.5kg flywheel inside but it’s geared up to x9 by the time it hits this nylon gear so I want it to be a solid connection. It’s a 65mm diameter by 15mm wide helical gear, the shaft is 12mm diameter but could be increased to 15mm if that was needed. So far I’ve these three ideas:

advice on my first project.

OK.  I’m an absolute beginner with anything electronic but I’d like to have a go so I thought the best way to learn is to pick a simple project.