One of the common problems we encounter while installing a fish finder on our kayaks is the bolt hole mounting pattern that your particular model of fish finder uses may be a different size or bolt hole pattern than what you plan to secure the mount to. Alot of us kayak anglers use a Scotty #269 Fish Finder Mount, or a RAM Mount of some type while rigging our kayaks. Quite often, the mounting plate on the fish finder is larger that what you want to mount it to, and that is where making a fish finder adaptor plate comes into play.
Malibu Kayaks Extreme fishing kayak

Most are made out of very inexpensive plastic cutting boards that you can buy at Wal Mart or a similar store for under ten dollars, plus the cost of additional stainless steel hardware if needed. If you can fabricate simple items you can usually make an adapter plate in an hour or less and be able to mount your fish finder, GPS mount and more. In the photo, you can see the fish finder mount which is the same gray color as the fish finder head itself.

My particular base uses three mounting screws which I screwed to the top of the adaptor plate. Below that, you can see a white plastic adaptor plate with two screws in it which are visible in the photo. There are a total of four screws in it, two which cannot be seen in the photo. Those four screw holes were match drilled to match the pattern in the Scotty mounting plate which is secured to the removable post. When I want to install the fish finder unit, I insert the post in the Scotty #241 mount, rotate it then lock it into position. I then connect the electrical connector plug and am ready to go. To remove the unit is the reverse order.