I have recently found some very desirable unused original old-stock aluminium sound-box diaphragms. The first ones pictured are fluted and with a diameter of 55mm. They are made for Columbia boxes, I suspect, but could be readily adapted for many other sound-boxes.
The second ones are, the very hard to find, diaphragms for the HMV No.5B
sound-boxes and are complete with the “spider” attachment. These diaphragms are a real find for those prepared to undertake the tricky job of restoring these very good sound-boxes.