This is a very difficult filter to find. I too have the SK43mm but was unable to find the matching CF.
However, there are many copies of the CF for the 47 XL out there, which I was able to acquire. I then had SK Grimes custom make a retainer ring to hold the glass and fit onto the 43mm.
It works very well, albeit you do get a slight vignette in the corners at 30mm of shift.