I'm trying to use this plugin only for ONE post. And I did what you said, I created a custom field on that particular post.
That did work, and I thought all was well until I noticed that ALL the blog posts on the main blog page were affected, and all of them were unformatted! They looked terrible (without the correct line-height, no paragraph breaks, etc) so I had to disable the plugin, because for the life of me, I can't get it to work for only ONE post.
And I don't know what to do, because that one post keeps adding paragraph tags around it, which is messing up the way it's supposed to look.