Print the TinkerCAD parts, also available on thingiverse http://www.thingiverse.com/thing:1247482

Chose a layer height that divides into 0.5 (e.g. 0.5, 0.25, 0.1).

The parts will fit on a 150x100mm print bed, to fit on a smaller printer, you could split the base and lid in half and then glue the halves together.

You will need 4x m3 bolts/nuts to secure the steppers and 10x dupoint cable to hook it all up.

The Pi only has 2 5V pins, connect those to the motor drivers and either build a splitter or shove a male duopoint cable into the back of one of the female stepper motor ones (see picture).

You'll want to put some tape (e.g. kapton, duct) between the drivers.

You'll also want to add a 1k resistor between the 5v echo pin of the ultrasonic module and the 3v3 GPIO on the Pi. To save soldering you can strip another dupoint cable and wire wrap it onto the resistor.

To attach the rear wheel you may be able to use some 3mm fillament or just get a 3mm metal pin like I have. A bit of heat helps the metal pin fit if its tight.