Trying to change sidebar for some posts

For a minority of posts I want to use a different sidebar from the one I've set as default/main. I've already used the plugin Custom Widget Area to create a couple of alternative sidebar menus, which work fine for Pages (you can just select the sidebar you want from Edit Page). However, for some reason this option doesn't appear for Posts, where I have the same need. I thought I might get around this by using the Custom Fields facility, sidebar_replacement_id option, and point it at one of my existing alternative sidebars. But either this is just misguided, or I can't find the right value to point it at. The alternative non-theme sidebar identifiers are: Sn Name Theme location (Id) Short Code Full code Hoping someone can point me in the right direction! I'm trying not to edit php files & get into child theme territory - so looking for a theme- or plugin- based solution if poss. Thanks in advance.