Awesome idea, and they already exist...

http://www.eaieducation.com/530397.html

http://www.amazon.com/Advantus-Corporation-Rubber-Stamps-Clo...

Or stickers that you can keep in your notebook.

just use your pencil to draw one

Nah. I kinda like the clean circle right smack in the middle. I'm thinking right now about how can I retrofit this into my filofax planner. Very nice idea. Thanks for sharing the link.

Or you can buy / make a small drafting template:

http://www.cutting-mats.net/drafting-template-3441.html

Although the rubber stamp is cool, I don't really want to carry something extra.