Did some testing these last few days, by making this platform:
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the motor holders are made out of 3mm steel, if it turns out to be too heavy, I might make them out of HDPE.
The wheels are made using slices of go-kart tyres which I screwed them to the wheels:
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The go-kart rubber is very grippy! When I try to push the robot, the wheels don't slide but they turn the motors.
The problem is that leaving them this wide, the wheels aren't perfectly round, but they flex with the weight.

Also the scooter motor had arrived, and I will do some tests on that later.