Duo Discus canopy problems

I recently had the alarming experience of the canopy coming nearly fully
open on a Duo Discus shortly after take-off on aerotow. Both my student (who
was flying the glider at the time) and I grabbed the handles when the canopy
was about 45 degrees open and managed to close it. However all efforts to
lock it failed, probably because the airflow was distorting the Perspex and
preventing the frame from seating correctly, so we resorted to holding it
closed. The canopy catch has had a SH mod. kit fitted which is supposed to
apply some spring pressure to keep the catch in the closed position.

The takeoff in question was a relight after landing at another airfield, and
after discussion with my student, we decided to continue the launch and had
an otherwise uneventful glide home, holding the canopy shut till we landed.

Both my student and I clearly recalled doing our pre-takeoff checks,
including confirming that the canopy was locked closed. The most likely
cause of the incident seems to be vibration during the slightly bumpy
takeoff run, which probably caused the catch to move towards the open
position.

I subsequently discovered that this has happened before. The mod. kit is
pretty ineffective as the spring isn't even strong enough to pull the catch
into the closed position when the canopy is open, let alone when the
mechanism has to engage at three points along the canopy's length.

The following day I wrapped a single turn of PVC wing tape around the catch;
enough to hold it closed, but easy to break if the canopy needed to be
opened in an emergency. The worst aspect of the whole incident was losing my
favourite gliding hat which blew off my head when the canopy opened!

Has anyone had a similar experience or advice on improving the Duo's canopy
catch?

David Starer