This is a Gist courtesy of wpbeginner.com (http://www.wpbeginner.com/wp-themes/create-custom-single-post-templates-for-specific-posts-or-sections-in-wordpress/)

Custom Single Post Templates for Specific Category

/** To use a separate single post template for each category, you will need to add the following function in your functions.php: **/

/**
* Define a constant path to our single template folder
*/
define(SINGLE_PATH, TEMPLATEPATH . '/single');

/**
* Filter the single_template with our custom function
*/
add_filter('single_template', 'my_single_template');

/**
* Single template function which will choose our template
*/
function my_single_template($single) {
global $wp_query, $post;

/**
* Checks for single template by category
* Check by category slug and ID
*/
foreach((array)get_the_category() as $cat) :

if(file_exists(SINGLE_PATH . '/single-cat-' . $cat->slug . '.php'))
return SINGLE_PATH . '/single-cat-' . $cat->slug . '.php';

elseif(file_exists(SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php'))
return SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php';

endforeach;