Whilst there I found that the NFR (Nearly Flight Ready) model kits for gliders also were using brakes and that one of the few jobs that you do is to re-enforce the brake.
So my example is the shaping of some 6mm sq spruce spares to glue to the inside as in the following picture.
So now with this re-enforcing they close fully.
I also found out some more interesting information on the controls. The air brakes are normally connected to the throttle control and are therefore proportional in control.
Also my total aileron control length it about 50% of the wing width and on the NFR models its more like 25%. I wonder what that will do the flight control, might be very twitchy.