In your initialization file, bind the Transient casual-eww-tmenu to your key binding of preference.
(keymap-set eww-mode-map "C-o" #'casual-eww-tmenu)
Do so similarly to access casual-eww-bookmarks-tmenu in the EWW bookmarks list.
(keymap-set eww-bookmark-mode-map "C-o" #'casual-eww-bookmarks-tmenu)
While not mandatory, the following bindings can make the EWW keymaps consistent with those used by Casual.
(keymap-set eww-mode-map "C-o" #'casual-eww-tmenu) (keymap-set eww-mode-map "C-c C-o" #'eww-browse-with-external-browser) (keymap-set eww-mode-map "j" #'shr-next-link) (keymap-set eww-mode-map "k" #'shr-previous-link) (keymap-set eww-mode-map "[" #'eww-previous-url) (keymap-set eww-mode-map "]" #'eww-next-url) (keymap-set eww-mode-map "M-]" #'eww-forward-url) (keymap-set eww-mode-map "M-[" #'eww-back-url) (keymap-set eww-mode-map "n" #'casual-lib-browse-forward-paragraph) (keymap-set eww-mode-map "p" #'casual-lib-browse-backward-paragraph) (keymap-set eww-mode-map "P" #'casual-eww-backward-paragraph-link) (keymap-set eww-mode-map "N" #'casual-eww-forward-paragraph-link) (keymap-set eww-mode-map "M-l" #'eww) (keymap-set eww-bookmark-mode-map "C-o" #'casual-eww-bookmarks-tmenu) (keymap-set eww-bookmark-mode-map "p" #'previous-line) (keymap-set eww-bookmark-mode-map "n" #'next-line) (keymap-set eww-bookmark-mode-map "<double-mouse-1>" #'eww-bookmark-browse)