I use Maalit (or is it Maliit? I can never recall). It works and has an extended symbols button, just like a mobile phone osk. It doesn't have a proper configuration panel, it's instead configured by gsettings or something, I have never actually tried. It also as a quirk with a swipe-down gestures that is used to disable it, and is sometimes activated accidentally by the palm of the hand. But apart from that, really nice experience overall.