I know nothing about git.
However, this is how I managed to download the most recent version of openbox from git, and make it ready for the slackbuild script.
git clone git://git.openbox.org/mikachu/openbox
cd openbox and run
cd .. and rename openbox to the correct version, e.g.
tar cf openbox-6.2.tar openbox-6.2
openbox-6.2.tar.gz to slackbuilds builddir and open the slackbuild script and change the VERSION variable.