Sometimes people happen on to a decent old long board but it lacks a centerboard. Replacements can be difficult or expensive to find and making your own if you don't have the experience or tools can be a challenge.

This clever guy modified an old Formula fin as a replacement for an old Mistral board.
https://groups.io/g/nw-windtalk/message/72129
https://groups.io/g/nw-windtalk/message/72130

entire thread: https://groups.io/g/nw-windtalk/topic/75341533

"Formula" centerboard beside regular used as a pattern