A few questions:
What program are you using to make the sigs?
Are you resizing the models in the model viewer and transfering them to the program or are you resizing the models in the program? The models seem to have rough edges to them.
What usually works is to grab a bigger version of the model viewer and then reduce the size once the captured image is in the other program.
As for the boxes and colors, test out colors and see what looks best. Try for stuff not too contrasting and something to fit the style of the sig (not an art or graphics design major so correct me if my wording is off ;p).