A new idea, which was actually first idea, but I skipped it because I didn't think it would work:

It seems to slip at around 7 kg·cm. Since the motor can only provide 9 kg·cm of torque, I can probably get the rest by roughing up the motor shaft so that it's not smooth. I'll have to think about how best to do that.

The advantage to this is that once those sliding pieces are bolted into the middle, I have a solid 10 mm thick piece of plastic from the outside to the motor shaft, unlike with the last design where it has to be thin in order to leave room to attach the clamp. So it should make a more durable wheel.

Here's a photo of the first failed attempt to produce this, in which you can see the sloped interior of the hub.

The idea here is that, because the inside of the hub has a 45° slope, tightening the bolts wedges the six "pizza slice" pieces between the hub and the shaft, creating a tight friction fit.