Cheap Vehicle Data Logger

I have about 5 vehicles at work that do daily routes and I've gotten complaints that some of my drivers are speeding and what not. Is there a cheap data logging device that I can plug into the OBD2 port secretly and pull a week later and see what was going on?

I don't want something expensive that requires a monthly fee. I just want to pop it in at random, without the drivers knowing, and then download the results at the end of the week.