I intended to start cutting plastic today, but so far I've not managed it. The main set back has been finding that my stock of polycarbonate is 10mm, not 8mm as I'd designed around. The 10mm would add over 40grams to the weight and I was very worried it would make the design overweight. Attempts at lightening holes in the CAD showed that it was futile - they only saved 20grams even if I peppered it with 10mm holes :-(

I figured out a way to make it narrower though. If I mount the wheels onto the shafts the other way around then I could save 15mm off the width. That gets me 30grams back:

Left image shows the hub mounted on the deep side of the wheel with the motor shaft mounted on that side.
Right image shows the motor shaft coming in from the other side, recessing the gearbox into the wheel. I could save a bit more if I mounted the hub on the other side but I wouldn't be able to support the gearbox as easily then and actually other parts in the robot start becoming the limiting factor to its width.
After that change I realised that I could shave a bit off the length too. Just 10mm off the length saved another 30grams, so I'm now 20grams lighter than when I started and I've moved up to 10mm frame rails! Great! I just hope I can squeeze the wires and electronics in the space in the middle. Here's the latest CAD: