For a GUI approach I can only hint at Obkey. It seems to be abandoned, so it may not work. I haven't used it myself.
But if that doesn't work, fret not! I'll show you how to do it manually.
Lubuntu-rc is an XML file, it uses tags to define its structure. Each element has an opening and closing tag like this
<tag></tag> or like this
The Alt+Space combo is normally assigned to a ShowMenu action. To reassign the key combo, navigate to
~/.config/openbox and edit the
lubuntu-rc.xml config file using your editor of choice.
Search for the following element:
Yu must change /path/to/albert_bin/ to whatever is appropiate.
Finally save your changes and reload Lubuntu/Openbox.
Another neat feature is openbox chrooting, which allow to create simple mnemonic application launchers like this:
<keybind key="W-a" chroot="true">
<keybind key="Escape C-c BackSpace">
(add your key bindings here)
Pressing Windows/Meta+a ("a" for applications), one gets into the chroot, then it would be possible to launch apps by pressing single keys, for example, t for terminal. You need to add suitable key bindings for terminal and other apps.
To exit the chroot, press Escape, Backspace or Ctrl+c.
Check this out for more info on OB bindings and chroots.