I'm sure your local Yamaha dealer could point you in the right direction. The extentions are pricy. I have had the same problem with my cat. My engine
has 2 extensions and a 3rd would have been a custom deal. I lowered the transom mount a couple of inches and it helped a good deal. Some people swear by the fins you can put on outboards. However, I found that when I took them off it cavitated less. Good luck.