According to the following information on the KHB Photographix site the Fujinon EX 135mm f/5.6 enlarging lens requires a 53mm x 0.75 flange or retaining ring:
http://store.khbphotografix.com/Fujinon-EX-135mm-f5.6-Enlarging-Lens-for-4-x5-Negatives-Used.html
That size was also used by the original 150/5.6 EL Nikkor enlarging lenses and the 240/9 and 180/9 APO Nikkor process lenses. This makes it more likely that this size mounting flange or retaining ring could be found used.
The Omega 421-915 lens plate is threaded 53mm x 0.75 for mounting lenses of that thread on Omega D5/D6 enlargers. This plate can be easily mounted to a 4” x 4” Beseler lens board by drilling and tapping a couple of #6-32 screws to mount the Omega plate to the Beseler lens board. The two small holes in the photo given in the following link of the Omega plate are clearance holes for #6 screws to pass through. The Omega 421-915 plate can be found used on eBay from time to time.
http://www.khbphotografix.com/omega/LensMountD5.htm
Possibly KHB Photographix could help you with a mount for your lens.
I suggest measuring the outside diameter with a micrometer or dial caliper to be certain of the actual thread diameter of your lens and try to acquire the appropriate sized flange, threaded plate, or retaining ring.