It must be the "open cell" foam type (not the typical pillow cushion type or the rubber type).
It must be somewhat compressed to seal it down to board level or air can get in underneath, between foam and board.
It also must be the correct thickness for that type of foam. Too thin and wind gets through and you get same erratic results as with no foam. Too thick (and compressed) and air gets through too slowly and you get slow large oscillation.
Starting with no foam so you know what it looks like flying, keep making it thicker till you are satisfied.
Google some images. It is usually dark gray.
Some web sites also recommend a cotton ball held on with a wrap of tape.