Kevin mentioned that the slots are currently 2mm and this might be too tight a fit. After putting it together it seems that another mil or two is needed to avoid the fraying as seen in the below image:
The tight fit means quite a bit of force needs to be applied to get the thing to snap together. This causes fraying which is a downside but the upside is it holds together really well. If you don’t mind the fraying we could stick with 2mm.
The Breadboard window
The window is the perfect size for accessing the breadboard. Access to wires/components through the window is quite good especially if a pliers is used. Also, sliding out the breadboard is quick and easy if needed. The current design doesn’t really have any space for the battery pack unless you want to zip tie it to the underside of the robot. This is not a great solution because the breadboard would then have to sit on top of the zip ties. Cutting out an extra section from the breadboard window seems to work quite well because the battery pack can sit in it and be lifted in and out with ease. This removes the three holes near the back of the robot but these could be made smaller and moved somewhere else if they are really needed.
Drawing of modifications with dimensions:
(click on images to bigify)
In the right image, the 210mm dimension should be 21mm as in 2.1cm.
If the wheels are around 7cm in diameter, then a front wheel radius of around 2.5 cm is the best fit. (This was the middle one out of the three provided cut outs). The extra support bar on the front wheel is probably needed. The current design is pretty flimsy and the top platform tends to slip downwards.