Modified version of "class-wc-boxpack.php" version "3.2.18". See: https://gist.github.com/RadGH/7c8f0ef77e242ce011932d41e9c06b8e

class-wc-boxpack.php

<?php

/**
 * WooCommerce Box Packer
 *
 * @version 2.0.1
 * @author WooThemes / Mike Jolley
 */
class WC_Boxpack {

	private $boxes;
	private $items;
	private $packages;
	private $cannot_pack;

	/**
	 * __construct function.
	 *
	 * @access public
	 * @return void
	 */
	public function __construct() {
		include_once( 'class-wc-boxpack-box.php' );
		include_once( 'class-wc-boxpack-item.php' );
	}

	/**
	 * clear_items function.
	 *
	 * @access public
	 * @return void
	 */
	public function clear_items() {
		$this->items = array();
	}

	/**
	 * clear_boxes function.
	 *
	 * @access public
	 * @return void
	 */
	public function clear_boxes() {
		$this->boxes = array();
	}

	/**
	 * add_item function.
	 *
	 * @access public
	 * @return void
	 */
	public function add_item( $length, $width, $height, $weight, $value = '', $meta = array() ) {
		$this->items[] = new WC_Boxpack_Item( $length, $width, $height, $weight, $value, $meta );
	}

	/**
	 * add_box function.
	 *
	 * @access public
	 * @param mixed $length
	 * @param mixed $width
	 * @param mixed $height
	 * @param mixed $weight
	 * @return void
	 */
	public function add_box( $length, $width, $height, $weight = 0 ) {
		$new_box = new WC_Boxpack_Box( $length, $width, $height, $weight );
		$this->boxes[] = $new_box;
		return $new_box;
	}

	/**
	 * add_package function.
	 *
	 * @access public
	 * @param mixed $length
	 * @param mixed $width
	 * @param mixed $height
	 * @param mixed $weight
	 * @param mixed $value
	 * @return void
	 */
	public function add_package( $length, $width, $height, $weight = 0, $value = 0 ) {
		$package           = new stdClass();
		$package->id       = '';
		$package->weight   = $length;
		$package->length   = $width;
		$package->width    = $height;
		$package->height   = $weight;
		$package->value    = $value;
		$package->unpacked = true;
		
		$this->packages[]  = $package;
	}

	/**
	 * get_packages function.
	 *
	 * @access public
	 * @return void
	 */
	public function get_packages() {
		return $this->packages ? $this->packages : array();
	}

	/**
	 * pack function.
	 *
	 * @access public
	 * @return void
	 */
	public function pack() {
		try {
			// We need items
			if ( sizeof( $this->items ) == 0 ) {
				throw new Exception( 'No items to pack!' );
			}

			// Clear packages
			$this->packages = array();

			// Order the boxes by volume
			$this->boxes = $this->order_boxes( $this->boxes );

			if ( ! $this->boxes ) {
				$this->cannot_pack = $this->items;
				$this->items       = array();
			}

			// Keep looping until packed
			while ( sizeof( $this->items ) > 0 ) {
				$this->items       = $this->order_items( $this->items );
				$possible_packages = array();
				$best_package      = '';

				// Attempt to pack all items in each box
				foreach ( $this->boxes as $box ) {
					$possible_packages[] = $box->pack( $this->items );
				}

				// Find the best success rate
				$best_percent = 0;

				foreach ( $possible_packages as $package ) {
					if ( $package->percent > $best_percent ) {
						$best_percent = $package->percent;
					}
				}

				if ( $best_percent == 0 ) {
					$this->cannot_pack = $this->items;
					$this->items       = array();
				} else {
					// Get smallest box with best_percent
					$possible_packages = array_reverse( $possible_packages );

					foreach ( $possible_packages as $package ) {
						if ( $package->percent == $best_percent ) {
							$best_package = $package;
							break; // Done packing
						}
					}

					// Update items array
					$this->items = $best_package->unpacked;

					// Store package
					$this->packages[] = $best_package;
				}
			}

			// Items we cannot pack (by now) get packaged individually
			if ( $this->cannot_pack ) {
				foreach ( $this->cannot_pack as $item ) {
					$package           = new stdClass();
					$package->id       = '';
					$package->weight   = $item->get_weight();
					$package->length   = $item->get_length();
					$package->width    = $item->get_width();
					$package->height   = $item->get_height();
					$package->value    = $item->get_value();
					$package->unpacked = true;
					$this->packages[]  = $package;
				}
			}

		} catch ( Exception $e ) {

			// Display a packing error for admins
			if ( current_user_can( 'manage_options' ) ) {
				echo 'Packing error: ',  $e->getMessage(), "\n";
			}
    	}
	}

	/**
	 * Order boxes by weight and volume
	 * $param array $sort
	 * @return array
	 */
	private function order_boxes( $sort ) {
		if ( ! empty( $sort ) ) {
			uasort( $sort, array( $this, 'box_sorting' ) );
		}
		return $sort;
	}

	/**
	 * Order items by weight and volume
	 * $param array $sort
	 * @return array
	 */
	private function order_items( $sort ) {
		if ( ! empty( $sort ) ) {
			uasort( $sort, array( $this, 'item_sorting' ) );
		}
		return $sort;
	}

	/**
	 * order_by_volume function.
	 *
	 * @access private
	 * @return void
	 */
	private function order_by_volume( $sort ) {
		if ( ! empty( $sort ) ) {
			uasort( $sort, array( $this, 'volume_based_sorting' ) );
		}
		return $sort;
	}

	/**
	 * item_sorting function.
	 *
	 * @access public
	 * @param mixed $a
	 * @param mixed $b
	 * @return void
	 */
	public function item_sorting( $a, $b ) {
		if ( $a->get_volume() == $b->get_volume() ) {
	        if ( $a->get_weight() == $b->get_weight() ) {
		        return 0;
		    }
		    return ( $a->get_weight() < $b->get_weight() ) ? 1 : -1;
	    }
	    return ( $a->get_volume() < $b->get_volume() ) ? 1 : -1;
	}

	/**
	 * box_sorting function.
	 *
	 * @access public
	 * @param mixed $a
	 * @param mixed $b
	 * @return void
	 */
	public function box_sorting( $a, $b ) {
		if ( $a->get_volume() == $b->get_volume() ) {
	        if ( $a->get_max_weight() == $b->get_max_weight() ) {
		        return 0;
		    }
		    return ( $a->get_max_weight() < $b->get_max_weight() ) ? 1 : -1;
	    }
	    return ( $a->get_volume() < $b->get_volume() ) ? 1 : -1;
	}

	/**
	 * volume_based_sorting function.
	 *
	 * @access public
	 * @param mixed $a
	 * @param mixed $b
	 * @return void
	 */
	public function volume_based_sorting( $a, $b ) {
		if ( $a->get_volume() == $b->get_volume() ) {
	        return 0;
	    }
	    return ( $a->get_volume() < $b->get_volume() ) ? 1 : -1;
	}

}