Ok, first things first, what plastic are you using? Folks around here tend to suggest HDPE, it's super resistant to impacts and can be machined with basic woodworking tools (that is, saws and bits intended for wood). What batteries and motors did you order? Could use knowing what you've already got before commenting. For me personally, finding adequate wheels to fit to the motor shafts was a problem, you'll want to give that some thought.