WordPress Change custom taxonomy metabox (checkboxes to radios, in order to force one)

wordpress-hooks-metabox.php

<?php
/* Custom Tax Metabox : remove the current custom taxonomy metabox
/* -------------------------------------------------- */
add_action( 'admin_menu', function(){
	remove_meta_box('customtaxdiv', 'posttype', 'normal');
});

/* Custom Tax Metabox : add a new & improved custom taxonomy metabox (select-one)
/* -------------------------------------------------- */
add_action( 'add_meta_boxes', function() {
	add_meta_box( 'customtaxdiv_id', 'Title metabox', 'customtax_metabox', 'posttype', 'side', 'core');
});

function customtax_metabox( $post ) {
	//Get taxonomy and terms
	$taxonomy = 'customtax';

	//Set up the taxonomy object and get terms
	$tax = get_taxonomy($taxonomy);
	$terms = get_terms($taxonomy,array('hide_empty' => 0));

	//Name of the form
	$name = 'tax_input[' . $taxonomy . ']';

	//Get current and popular terms
	$popular = get_terms( $taxonomy, array( 'orderby' => 'count', 'order' => 'DESC', 'number' => 10, 'hierarchical' => false ) );
	$postterms = get_the_terms( $post->ID,$taxonomy );
	$current = ($postterms ? array_pop($postterms) : false);
	$current = ($current ? $current->term_id : 0);
	?>

	<div id="taxonomy-<?php echo $taxonomy; ?>" class="categorydiv">

		<!-- Display tabs-->
		<ul id="<?php echo $taxonomy; ?>-tabs" class="category-tabs">
			<li class="tabs"><a href="#<?php echo $taxonomy; ?>-all" tabindex="3"><?php echo $tax->labels->all_items; ?></a></li>
			<li class="hide-if-no-js"><a href="#<?php echo $taxonomy; ?>-pop" tabindex="3"><?php _e( 'Most Used' ); ?></a></li>
		</ul>

		<!-- Display taxonomy terms -->
		<div id="<?php echo $taxonomy; ?>-all" class="tabs-panel">
			<ul id="<?php echo $taxonomy; ?>checklist" class="list:<?php echo $taxonomy?> categorychecklist form-no-clear">
				<?php   foreach($terms as $term){
					$id = $taxonomy.'-'.$term->term_id;
					echo "<li id='$id'><label class='selectit'>";
					echo "<input type='radio' id='in-$id' name='{$name}'".checked($current,$term->term_id,false)."value='$term->term_id' />$term->name<br />";
					echo "</label></li>";
				}?>
			</ul>
		</div>

		<!-- Display popular taxonomy terms -->
		<div id="<?php echo $taxonomy; ?>-pop" class="tabs-panel" style="display: none;">
			<ul id="<?php echo $taxonomy; ?>checklist-pop" class="categorychecklist form-no-clear" >
				<?php   foreach($popular as $term){
					$id = 'popular-'.$taxonomy.'-'.$term->term_id;
					echo "<li id='$id'><label class='selectit'>";
					echo "<input type='radio' id='in-$id'".checked($current,$term->term_id,false)."value='$term->term_id' />$term->name<br />";
					echo "</label></li>";
				}?>
			</ul>
		</div>

	</div>
	<?php
}

/* Custom Tax Metabox : add some JS to keep it functioning right
/* -------------------------------------------------- */
add_action('admin_enqueue_scripts', function() {
	wp_register_script( 'radiotax', get_template_directory_uri() . '/js/radiotax.js', array('jquery'), null, true );
    // We specify true here to tell WordPress this script needs to be loaded in the footer
	wp_enqueue_script( 'radiotax' );
});

wordpress-radiotax.js

jQuery(document).ready(function($) {
	var taxonomy = 'customtax';
	$('#' + taxonomy + 'checklist li :radio, #' + taxonomy + 'checklist-pop :radio').live( 'click', function(){
		var t = $(this), c = t.is(':checked'), id = t.val();
		$('#' + taxonomy + 'checklist li :radio, #' + taxonomy + 'checklist-pop :radio').prop('checked',false);
		$('#in-' + taxonomy + '-' + id + ', #in-popular-' + taxonomy + '-' + id).prop( 'checked', c );
	});
});