(defun my-slugify (str)
  (downcase
   (replace-regexp-in-string
    "[[:space:]-]+" "-"
    (my-trim-string
     (replace-regexp-in-string
      "[^[:word:][:space:]]+" "" str)))))

(defun my-trim-string (string)
  (replace-regexp-in-string
   "\\`[ \t\n]*" ""
   (replace-regexp-in-string "[ \t\n]*\\'" "" string)))

(setq elfeed-show-enclosure-filename-function
      (lambda (entry url-enclosure)
        (let*
            ((fname (file-name-nondirectory
                     (url-unhex-string
                      (car (url-path-and-query (url-generic-parse-url
                                                url-enclosure))))))
             (feed-title (elfeed-feed-title (elfeed-entry-feed entry))))
          (mapconcat 'directory-file-name (list (my-slugify feed-title) fname) "/"))))