Adding an information box to a post

Do you want it to be part of the post, or have the same text box show up at the bottom of every post?

So instead of doing a widget or template modification, I would just use a div, styled with CSS to make it look however you want. Then copy it into the post content for each post where you want it to appear. Give the div a class, and then define the class attributes in your style sheet.

Here’s a good tutorial to start with:

http://gostats.com/resources/css-div-tutorial.html