Use a template other than single.php

The plugin uses single.php to display a calendar. I’d like to keep single.php as-is (for my blog) and use a slightly edited clone of single-php for calendars. As I understand it, one would usually do this by appending the custom post type slug to the cloned template’s file name, something like single-simple_calendar.php. Have I got that right? That particular file name — single-simple_calendar.php — isn’t recognized by the plugin, presumably because I haven’t correctly guessed the custom post type slug. What’s the slug? And if I use the correct slug, will the plugin use the new template instead of single.php?