Plugin to output a basic page for iframe

I have a site used to create some “advanced content” like forms and it need to be “iframed” in another site. I was searching for a plugin to choose a different theme for a page and a stripped theme to be activated there; but I think a better approach would be to create a plugin which for https://www.site.com/iframed/12 returns a stripped version of the page 12.

Does some plugin like this exists? Or any other approach to solve the problem is welcome.

Thanks