Airspeed indicator for $99
A static port is a must. A change in altitude of 500 ft is the equivalent change in pressure seen at aprox 80 mph.
The problem with the "Pilatus Porter" setup was probably due to the effect of air flow around unsealed portions of the fuselage. These openings are even likely to be affected by prop wash. A test to determine if this is the case might be to collect data with the engine at different rpm while the aircraft is stationary.
If you have a chance to try this ( or convince the guy with the setup to do it) let me know the results. It would be valuable information.