The
blank sidebox plugin does what you're asking, i.e. provides a template from which you can develop your own sideboxes that will fit with the styling of your other sideboxes. It says that it's for 1.3.7 only because that's the last time it was updated - the way sideboxes are built hasn't changed since then.
You may also be interested in the
editable sidebox plugin, which makes the content editable from the admin in the same way as it is for EZ-Pages or Define pages.
You can change the width of your sideboxes by editing values in Your-Admin > Configuration > Layout Settings.
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