Download fluxbox source code from its official website.
http://fluxbox.org/download/

sudo apt-get install build-essential texinfo libacl1-dev autoconf automake xorg-dev libx11-dev libimlib2-dev libxinerama-dev libxrender-dev libxft-dev libxpm-dev
./configure --prefix=/usr --enable-nls --enable-imlib2 --enable-xinerama
make
sudo make install

Use you’re start up file. Type nano ~/.fluxbox/startup (if it does not exist, create it) and put this in there

if [ -z "$DBUS_SESSION_BUS_ADDRESS" ]; then
  eval `dbus-launch --sh-syntax --exit-with-session`
fi
nm-applet --sm-disable &
pactl set-sink-volume 0 100%
exec /usr/bin/fluxbox

cd /usr/share/xsessions, create a new fluxbox.desktop, sudo rights required

[Desktop Entry]
Encoding=UTF-8
Name=Fluxbox
Comment=This is fluxbox
Exec=/home/(username)/.fluxbox/startup

With dual monitor setup, use the below to combine images and use fbsetbg to set it as background photo.

sudo apt-get install eterm
montage jordan1.jpg jordan2.jpg -geometry +0+0 jordan.jpg
fbsetbg -f jordan.jpg

Install eterm to resolve fbsetbg problem

References

article 1 https://linux.byexamples.com/archives/328/compile-fluxbox-under-ubuntu/

article 2 https://ubuntuforums.org/archive/index.php/t-116759.html

article 3 regarding network connectivity https://www.linuxquestions.org/questions/slackware-14/setting-up-network-manager-in-fluxbox-882892/

article 4 montage desktop wallpaper https://www.linuxquestions.org/questions/linux-software-2/using-montage-to-place-two-images-side-by-side-as-a-single-image-4175616132/