They don't mount to the fuse sides. They should mount to a horizontal plane such as the bottom of a pipe tunnel. Sticky two sided servo tape works.
That is right, it must be mounted horizontally. Reason for such orientation is, that wing is relatively stable, while profile fuselage (but also thin wall fuselage side) vibrates, and that is not any good place for such sensor. We use them also on indoors with thin fuselages from 3mm foam, wing is the only rigid place for them and it works well also on such construction. That is also reason why we do not fix it by screws, adhesive foam is good way for mounting.
Another problem is, that it should be close to CG and fuselage does not have too much space around thick wing for verticaly oriented sensor anyway.