Custom in-post ads by specific blog post


I like the idea of In-Post Ads but I can't get it working quite the way I want. Here's what I'm after...

I want to create an opt-in box as my ad. This ad will only be displayed on one particular blog post.

I would like to repeat this a number of times...i.e. creating specific opt-ins that directly relate to the content in the blog post where they will be placed.

How can I choose an in-post ad to be shown on only one specific blog post?