The Shapeways-model looks good. The side walls are strengthened at the lower end with an L-shape.
For the cleaning, I use 2% Sodium hydroxide (also known as lye and caustic soda, NaOH) at an ultrasonic cleaner. Multiple times of 20min with breaks between it. Because of the self-heating of the liquid at the ultrasonic-cleaner. After that, I clean it with water, soap and a toothbrush. Then rinse with clear water and drying at 40° Celsius. I use always Tamiya grey primer on all sides of the warm model, inner and out.
My suggestion for the drive chassis are
9' MTB-bogies at a brass-plate which fit inside. They have the right side frames. Two Stop points out of plastic over the whole inside width for the brass-plate can be glued probably inside the shell, above the L-shape. That the plate doesn't fall inside. The screws of the coupler boxes go thru the plate into the stop points and holds all in place