We have had blank slot flushers made which match the original Mistral material.
They are 902mm long and 70mm wide.
You will need to cut to the right length, then sand bottom (the side which is already slightly rough) to prepare for gluing.
Then drill the little holes at each end of where the slot should be which act as crack stoppers, and very carefully cut the slit for the dagger with a Stanley Knife. (A good tip is to tape the slot closed with masking tape or "Duck tape" on the outside before gluing, as this will ensure a nice tight slot flusher gap when it is glued on)
Then having made sure that the slot flusher is sanded and thoroughly clean, and the surface of the board is the same, apply glue to the board (use enough to ensure the surface to be bonded is completely covered, which will be at least half a 20 ml bottle) and place slot flusher on it, applying light pressure for at lease half an hour to ensure glue has taken properly - this is best done by using 2 wooden battens 18 to 20mm wide x 3 to 4 mm thick, cut to just inside the length of the slot flusher.
These should be weighted down for this period of time to ensure a good bond - 4 to 5 house bricks (on edge) are ideal items to use which are readily available (you need 8 to 10 bricks in total).
Balance the bricks on edge along the full length of each batten, touching at their bases over the centre line of the flusher, to ensure constant pressure is applied while the glue hardens (masking tape or similar will be required to stop the bricks falling outwards, as there is a slight angle to the surfaces the flusher is glued onto, which is enough to make the bricks want to fall beyond the point where they apply even pressure across the width of the battens) - the bricks must be secured to keep pushing at this angle in order to keep the battens applying even pressure - Leave to fully harden for at least 24 hours before use
All surfaces must be very clean and free of dust or Silicon contamination. Failure to do so could affect the bond of the flusher to the board!
Here is the link to the best glue for sticking these slot flushers down with.