Glad you had another go with sbbuilder, once you get the hang of it, it really beats the manual method.
Fontforge does have both a .desktop file and .png icon included in the source.
In the SlackBuild script generated by sbbuilder is a section that looks like this...
#mkdir -p $PKG/usr/share/applications
#cp $CWD/$NAME.desktop $PKG/usr/share/applications/$NAME.desktop
#mkdir -p $PKG/usr/share/pixmaps
#cp $CWD/$NAME.png $PKG/usr/share/pixmaps/$NAME.png
As you have probably guessed just by looking at it the comments '#' need to be taken out inorder for it to work.
Once that is done you should be able to re-run the script and have the .desktop file and png included in your package.
At least that is what you would think. I have found it is not so dependable, but the fix is pretty easy.
1st extract fontforge.tar.gz so you can get at the files inside and copy the .png and .desktop file to a place you can easily reference.
2nd. Change the '$CWD/$NAME.desktop' part to the actual path where you have copied the .desktop file to your drive.
3rd. do the same for the .png.
You should have something that looks like this...
mkdir -p $PKG/usr/share/applications
cp /home/granny/fontforge.desktop $PKG/usr/share/applications/$NAME.desktop
mkdir -p $PKG/usr/share/pixmaps
cp /home/granny/fontforge.png $PKG/usr/share/pixmaps/$NAME.png
Save your changes.
Delete your previous .tlz, slack-desc file, and slack-required file and run the fontforge.SlackBuild script again.
Let me know if it works.