Using the :set to make an alist

I want a customisable variable that is an alist with elements (key . directory-path).

Have made the :set function to set the list by appending sub-directories using orellana-fpln base directories. Because usually one uses (set-default symbol value) to set the value, I do not understand how this can be applied to a list.

 (defcustom galaxiakos-fpln nil
  "Association List (alist) specifying a flight plan, where each element is
a cons cell (WAYPT . DIRECTORY-PATH) linking a waypoint code to its
associated directory path for different Galaxiakos components."
  :group 'galaxiakos
  :type  '(alist :key-type (symbol :tag "Code")
                 :value-type (string :tag "Path"))
  :set  (lambda (symbol value)
          (set-default symbol value)
          (let ( (glxks-waypt  (assoc-default 'GLXKS orellana-fpln) )
            (when glxks-waypt
              (setq galaxiakos-fpln
                     `((GLXKS . ,glxks-waypt)
                       (OMNIX . ,(concat glxks-waypt "/omniskon"))
                       (PROTL . ,(concat glxks-waypt "/protilus"))
                       (GRGRS . ,(concat glxks-waypt "/gregers"))) )) )) )