This repo holds slackbuilds for a current version of Flatpak as well as current versions of dependencies that are not in the current Slackware base system.
This flatpak-tree is compatible with my slackgrab.py utility.
mkdir -p ~/slackstack/
cp -r flatpak-tree ~/slackstack/
Then you can use slackgrab.py to populate each of the slackbuild folders with the appropriate tarballs, and verify the md5sums.
Flatpak allows easy installation of software that is either missing from the Slackware repos or difficult to install. Examples include Libreoffice, Steam, VLC, Spotify and so on. See flathub.org for available titles.
Once everything is installed you simply
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
and you're off to the races. flatpak search to find software, flatpack install to install, etc. See man flatpak for details.
If Rofi doesn't list flatpak installs, try changing the key binding to rofi -show drun