Yeah . . funny you should say that. When I was a kid I flew a friends Kwik Fli Mk3 and noticed it wiggled. I later built one and didn't fit a canopy (just couldn't be bothered) and it seemed OK . . A lot of years ago, though.

Hmmm . . wonder if the canopy is deflecting airflow or causing turbulance over the fin ?? Maybe fit a T-Canaliser

Cheers, JB