For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:adamk/openSUSE_Tumbleweed/home:adamk.repo
zypper refresh
zypper install fluxbox
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:adamk/openSUSE_Slowroll/home:adamk.repo
zypper refresh
zypper install fluxbox
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:adamk/15.6/home:adamk.repo
zypper refresh
zypper install fluxbox