A motion detector shouldn't be very expensive anymore. Don't know about the US, but here in Europe you can get intruder alarm detectors in every DIY-store for very low prices.
Most cheap detectors use Passive Infra Red. These pick up the infra red light from a body that moves. Normaly, you'd have to actualy walk a step or two to trigger these things. Waving your hand shouldn't trigger it. PIR's are most sensetive for movement across (sideways) the detector.
These detectors idealy should be located near the top of your cab. What is a bit sad is that those detectors are usualy white and also the lens or lens-protector will be white. You can paint the housing, but not the lens protector.
Alternatively, use an UltraSonic detector. These can be placed lower. They are most sensitive for movement to and from the detector. However, these are much more uncommon. They were used a lot in car alarms. The beaty of us-detectors is that you could place it inside your cab, and drill very small holes for the transmitter and receiver. This would make it virtualy invisible. There are also DIY-electronics kit US detectors.
Whatever detector your get, be sure it has a common (C) and Normaly Open (NO) contact. Usualy in burgler alarms, normaly closed contacts are used for security reasons, but we don't need that, we want to simulate a key press (closing contact).
I am currently busy painting my home walls (got to keep the wife happy

)but will experiement with some detectors asap.