There are no Fluxbox dialog boxes and there’s no need for that, in your distribution’s package repository look out for:
Pretty sure there are even more options.
Use one of these with a bash script and call it from the Fluxbox menu or with a keyboard shortcut.
bash script: yad shutdown menu
#! /bin/bash
action=$(yad --width 300 --entry --title "System Logout" \
--window-icon=gnome-shutdown \
--image=gnome-shutdown \
--button="gtk-ok:0" --button="gtk-close:1" \
--text "Choose action:" \
--entry-text \
"Logout" "Reboot" "Power Off")
ret=$?
[[ $ret -eq 1 ]] && exit 0
case $action in
Logout*) cmd="killall fluxbox" ;;
Reboot*) cmd="sudo /sbin/reboot" ;;
Power*) cmd="sudo /sbin/poweroff" ;;
*) exit 1 ;;
esac
eval exec $cmd
zenity --list --text "System Shutdow" --radiolist --column "" --column "" TRUE shutdown FALSE reboot FALSE suspend FALSE hibernate --hide-header --title "System Shutdown"