This first shot shows the original internal baffle, which I have folded in the middle to shorten it, temporarily attached with peelable tape. I folded the middle section (which is double thickness into 3. This allows some light from the flash & reflector to leak directly to the ends of the front diffuser.
Removing the inner diffuser (now lying on the bottom) shows the prism reflector in place.
and here is a shot with the prism reflector also removed. The flash is a Canon 270Ex II sitting on top of a pixel king, and set to tele mode (front element pulled forward). I'll give it a go with some other flashes ion a day or 2 to see how it behaves.
The prism is 2 pieces of card 10cm wide the joined edges are 15.5cm long (which allows it to fit snuggly up against the hot shoe mount ring) and the edges nearest the front of the box are 17cm. They are at an angle of 85 degrees.
It's held in place for now with a couple of small strips of blu-tack to the top and bottom faces.
I made the prism with card covered in aluminium foil - probably easier to use silver mirror card if you have it to hand _ did think it might be necessary to crumple the foil slightly, but this doesn't seem to be needed.