The box

The box should be lightproof. Otherwise you'll see ambient light from the holes instead of pictures.

It's a good idea to wrap the box into grounded tin foil.

The duct tape finish on my pictures is to protect that from the elements.
If you don't isolate outside EMI, all you'll see in civilized areas is mains hum, especially with pA-level amplification:

And don't repeat my mistakes - mount the axes perpendicular to each others, not skewed.

The robot

Basically, you need a two-axis moving platform. One axis has to be fast (or you'll have to be patient), and will hold the sensor on it. The other axis can be as slow as comfortable, and will move the fast axis.
I used a dead scanner as the slow axis:

And a stepper with the belt directly attached as the fast axis:

The power

Make sure you have a battery that can handle the load with time to spare.
If the power start to drop, you will get changes and degradation across the image.

8 AA batteries, full on the right, scans to the left as they die:

While a UPS lead-acid battery supplies a steady current all the time:

Noise issues on the next page