defcustom manipulation

Could I include a let binding to use a variable to store the value of 
(assoc-default 'GLXKS orellana-fpln)?

What should I put in the function galaxiakos-fpln-update?
This defcustom variable sets an alist of elements (key . dir)

(defcustom galaxiakos-fpln
  `((GLXKS . ,(assoc-default 'GLXKS orellana-fpln))
    (OMNIX . ,(concat (assoc-default 'GLXKS orellana-fpln) "/omniskon"))
    (PROTL . ,(concat(assoc-default 'GLXKS orellana-fpln) "/protilus"))
    (GRGRS . ,(concat (assoc-default 'GLXKS orellana-fpln) "/gregers")))
  "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 #'galaxiakos-fpln-update)