{{{1}}}
Usage: {{sigbox-raw|Text}} to put the text in a box.
You can try {{sigbox-raw|{{User:Username/sig}}}} to put a template-ized sig inside the box, but {{sigbox}} is simpler.
One use for this could be to put <noinclude>{{sigbox-raw|1=</noinclude> and <noinclude>}}</noinclude> around your sig in the template itself (so you can easily see its size in Show Preview). The 1= bit protects it from being confused by any = signs in the sig (it still will get confused if you have any | signs outside of links).