If you are used to Ctrl+Alt+L you can add these lines somewhere betwen the <keyboard> tags in the ~/.config/openbox/trisquel-mini-rc.xml file:

<keybind key="C-A-L">      
  <action name="Execute">        
    <command>dm-tool lock</command>      
  </action>    
</keybind>

If you would rather use Super+L:

<keybind key="W-L">      
  <action name="Execute">        
    <command>dm-tool lock</command>      
  </action>    
</keybind>

You can also add both, and use either shortcut depending on your mood.