Another workaround is to hack the theme itself. For the Ambiance theme edit /usr/share/themes/Ambiance/metacity-1/metacity-theme-1.xml and in the section starting
change left_width, right_width and bottom_height from 1 to 3.
I have not tried it on the Radiance theme but I expect the same change would work there. Keep a note of what you have done as if the theme is updated you might have to do it again.