I would fore go the extension. The risk is to high that damage can occur.
I had a mare that had been ultra-sounded several times with an extension. Very calm mare. No problems. Then one day the vet ultra-sounded her with the extension and for some reason she jumped. Had to rush her to the LSU Veterinary Hospital. The extension had made a tear in her colon. She also had a 1 month old colt on her side. Had to remove the colt. She ended up with a colostomy bag for a while until the inside of her colon healed. It was touch and go for a while. There was also the risk of her insides becoming contaminated with feces and becoming septic. She spent 6-8 weeks in the hospital. It cost me several thousands of dollars.
You don't need an extension. Even a 28-29" mare can be ultrasound by hand. You just have to go slow and take your time. It can be a tight squeeze regardless whether the mare is 29" or 34".
By the way, after that incident, the vet no longer used an extension again.