There is a slackpack in the /extras dir on disk 2 or in the extra's dir on the FTP's. THere is also a slackpack available here -> http://fluxbox.bottino.com/
. THe one on the slack cd is a bit older. If you use a slackpack it should make a xinitrc.fluxbox in /etc/X11/xinit/ . If not, it's not to hard to make this file yourself. Make a copy of say gnomes entry, "cp xinintrc.gnome xinitrc.fluxbox" then open the xinitrc.fluxbox look at the very end of the file and delete the line about starting gnome and add the command to start flux instead. Mine is this "exec /usr/X11R6/bin/fluxbox" But mine is installed form the "official Slackpack" from disk2. Your install dir may vary.