How to save a checkbox meta box in WordPress?

I'm trying to add a checkbox into my custom meta box in WordPress and I ran into a problem with saving it - whenever I check the checkbox and update the post/page, it comes back unchecked again.

Here's the code I'm using:

add_meta_box(
    'sl-meta-box-sidebar',      // id
    'Sidebar On/Off',           // title
    'sl_meta_box_sidebar',      // callback function
    'page',                     // type of write screen
    'side',                     // context
    'low'                       // priority
);

function sl_meta_box_sidebar() {
    global $meta; sl_post_meta( $post->ID ); ?>
    <input type="checkbox" name="sl_meta[sidebar]" value="<?php echo htmlspecialchars ($meta['sidebar']); ?>" />Check to turn the sidebar <strong>off</strong> on this page.
}

This creates the checkbox in the sidebar of the "Edit Page" screen, as it should, no problem there. I'm not sure what should I enter in the value of the checkbox, with text fields it obviously returns whatever was saved as meta information... I tried just using "checked" instead cause that would be my first guess (then simply check for the value when using this meta data), but it didn't save the checkbox either.

Here's the function that saves all the meta data, which I assume causes this problem:

function sl_save_meta_box( $post_id, $post ) {
    global $post, $type;

    $post = get_post( $post_id );

    if( !isset( $_POST[ "sl_meta" ] ) )
        return;

    if( $post->post_type == 'revision' )
        return;

    if( !current_user_can( 'edit_post', $post_id ))
        return; 

    $meta = apply_filters( 'sl_post_meta', $_POST[ "sl_meta" ] );

    foreach( $meta as $key => $meta_box ) {
        $key = 'meta_' . $key;
        $curdata = $meta_box;
        $olddata = get_post_meta( $post_id, $key, true );

        if( $olddata == "" && $curdata != "" )
            add_post_meta( $post_id, $key, $curdata );
        elseif( $curdata != $olddata )
            update_post_meta( $post_id, $key, $curdata, $olddata );
        elseif( $curdata == "" )
            delete_post_meta( $post_id, $key );
    }

    do_action( 'sl_saved_meta', $post );
}

add_action( 'save_post', 'sl_save_meta_box', 1, 2 );

It works perfectly for text fields, but the checkbox just won't save. I'm not sure if the saving function is wrong, or am I missing something about the value of the checkbox.

Any help appreciated!

I had trouble with this previously and here is how I solved it.

First, creating the Checkbox.

<?php
function sl_meta_box_sidebar(){
    global $post;
    $custom = get_post_custom($post->ID);
    $sl_meta_box_sidebar = $custom["sl-meta-box-sidebar"][0]; 
?>

<input type="checkbox" name="sl-meta-box-sidebar" <?php if( $sl_meta_box_sidebar == true ) { ?>checked="checked"<?php } ?> />  Check the Box.
<?php } ?>

Next, saving.

<?php
add_action('save_post', 'save_details');

function save_details($post_ID = 0) {
    $post_ID = (int) $post_ID;
    $post_type = get_post_type( $post_ID );
    $post_status = get_post_status( $post_ID );

    if ($post_type) {
    update_post_meta($post_ID, "sl-meta-box-sidebar", $_POST["sl-meta-box-sidebar"]);
    }
   return $post_ID;
} ?>