You can go install this utility as an easy dpkg from my OBS, which includes a few of my own wrapper tools. One is a nice menu entry for rebuilding the fluxbox menus.
The nature of xdg menus is to use the system-wide /usr/share/applications directory and its contents, and also include your local ~/.local/share/applications (or other $XDG_DATA_DIRS/applications/ locations as defined), so using the system-wide xdg fluxbox menu would not be complete. Therefore, I highly recommend adding to your ~/.fluxbox/startup script:
/usr/bin/xdgmenumaker -f fluxbox -i > ~/.fluxbox/xdg-menu &
And adjust your ~/.fluxbox/menu file to include at least these entries:
[include] (.fluxbox/xdg-menu) #[include] (/etc/xdgmenumaker/fluxbox)
I recommend the above commented line, because in case for some reason your per-user stuff is broken,
The dpkg I build provides a menu entry for reloading it, so you don’t have to go to the command line and run the above command.