Chess Griffin has been working on bringing Mate desktop to Slackware users as another alternative besides KDE and XFCE and many other DE available on Slackware. Please note that this is not (yet) an official Slackware project. This is a personal project, but there's a possibility to have this included in Slackware if Pat decided to.
Mate is targeting to release 1.6 on April and Chess' script is now currently building 1.5.x releases. You can grab his packages on his GitHub repository. Read the README included and you can start building this packages.
This is a very fast pace project, so updates will be coming quite frequent during this development cycle, so please keep your eyes on the commits.