Boxes are not so usefull in this case, because each post would has to have its own content in those boxes, like text in one box, image in another, details in a third. The best thing would be a visual implementation of the grid inside a post, just like the site layout works, but I guess that's a feature request and perhaps not so easy to make.
Let's say I still want to use the secondary html content box plugin, because that's a real addition to making WP a CMS and extremely easy to use, what options are there? I can't create custom template in which I can add the secondary html content template code. The only thing that I could do is using the Secondary HTML Content widget in a sidebar. That would split the content not inside a post, but in the post and sidebar content. Just thinking out loud here