Add the PMPro meta box to a CPT. Add this to your plugin/etc.

pmpro-cpt.php

<?php
/*
	Add the PMPro meta box to a CPT
*/
function my_page_meta_wrapper()
{
	//duplicate this row for each CPT
	add_meta_box('pmpro_page_meta', 'Require Membership', 'pmpro_page_meta', 'product', 'side');	
}
function pmpro_cpt_init()
{
	if (is_admin())
	{
		add_action('admin_menu', 'my_page_meta_wrapper');
	}
}
add_action("init", "pmpro_cpt_init", 20);
?>

I can no longer edit gists made by the strangerstudios account. Here is a forked version that is simpler and actually works with the latest versions of WordPress and Paid Memberships Pro: https://gist.github.com/ideadude/59e4dffbfae2216cff2172ef140efc91