All depends on the fishfinder head unit.
My first recommendation would be a Ram ball mount. That way the only thing attached to your kayak is a Ram ball base. A double socket arm is then removable and the top end attaches to your head unit.
Second choice would be the YakAttack mounts. They call them LockNLoad, but they make several versions to fit a bunch of different fish finder models.
https://www.yakattack.us/products/depthfinder-mounting/ Trouble is they are all gear-track based. So you have to install gear track if you don't already have some. Then the entire fish finder head unit is trusted to the gear track. The Ram Ball solution is far stronger.
Those are the only two options I know of that I'd be willing to consider.
PM me if you want more info, I can likely be of some help