The chassis can be laser cut from 3 materials each with their own pro's and con's:

• Acrylic - Looks good and is affordable but extremely brittle and easy to break.
• Plywood - Cheap, strong and can easily be modified but some people think it looks less attractive.
• Polycarbonate - More expensive than acrylic but not brittle.

I personally would recommend using plywood as it is more robust and can easily be hacked by students wanting to modify their robots.

If you do not have access to a laser cutter there are several companies who offer laser cutting services for very reasonable prices online. I have included a ready made file for Razorlab. The design fits onto their P1 size sheets and will set you back ~£11.50 for a plywood E.R.N.I.E, although I am sure in bulk the price would decrease. Be sure to use 3mm thick material

I highly recommend getting professionally manufactured PCBs from one of the many Chinese PCB fab houses. I used Elecrow and got 50 boards produced for £22.57 (45p a board!). If you would like to recreate the exact boards I got made you can simply visit this page and enter the following settings:

• Layer - 2
• PCB Thickness - 1.6mm
• Copper Weight - 1oz 35um
• PCB Size - 10cm Max * 10cm Max
• PCB Color - Black
• Suface Finish - Hasl (lead free)
• PCB Stencil - NO Stencil
• Panelizing - 2-5 copies
• Lead time - Shipped in 4 to 7 days 

The zip file you need to attach can be found here

Each kit needs 3 wheels, two of which need to have their hub enlarged to 5mm. This can be done very easily on a drill press running on a slow speed or by hand with a drill bit.

Solder stranded wire onto the motors. The wires should be around 20cm long. If in doubt, cut the wire longer, you can always cut it down later.

Attach the motor to the mount using the (longer) 30mm screws:

Repeat this for the second motor but this time mirrored. The end result should look like this:

Attach the motor and axle mounts to the base plate using the 20mm screws:

NOTE: Do not over tighten the screws if using acrylic. Acrylic is a very brittle plastic and will crack easily if too much force is applied. Do not worry if the mounts have a little bit of movement in them, this will not affect the robot’s functionality.

Mount the Arduino to the base plate using 2 nuts as a spacer. Note that the front screw goes from the bottom and the back screw from the top:

Apply electrical tape around the rear wheel (the one with the smaller hole). This is so that the back wheel can easily slide around when the robot turns.

Attach the wheels to the robot:

You will find that the rear axle easily slips out of its mounts. To fix this simply wrap a little bit of electrical tape around the axle so it cannot slide around.