xlockmore is an enhanced version of xlock. It incorporates several new commandline options , which allow you to run it in a window, in the root window, in a different size/location, change the size of the iconified window, to install a new colormap and delay locking for use with xautolock.
9089db84a38d940c8efc42d7183198b9eb542928804e446207e6c28794a3261f