Using BMP180 on board a model airplane in order to measure altitude.

My question has nothing to do with the usage of the sensor but with the placement of the sensor.

I am afraid that If I place the sensor exposed to direct air, the relative wind that the plane is subjected will manipulate the results and make them incorrect. My plane will be traveling around 30 kmh or 18 mph.

Do I build an enclosure and punch a couple of hole on it? My airplane fuselage has not sealed space inside that I can put the sensor in. I just want to calculate my altitude relative to my take-off point.

At that speed I wouldn't worry too much. Stagnation pressure is proportional to velocity squared.
If there's no obvious funneling of air into the front or out the back of the fuselage it will be pretty
close I suspect.

You may see more noise in the measurements when moving though - I have a barometer app on my
phone and its clear from the trace when I've been in a car due to fluctuations (and also clear when
I've climbed or descended 3 flights of stairs at work!)

You can measure the discrepancy and add a correction factor.