Way to open virtual custom inventories in MCPE :D (Work in progress...) Special thanks to @alejandroliu and @PEMapModder for a previous research :3

VirtualInventories.php

<?php

/**
 * @name VirtualInventories
 * @main VirtualInventories\Loader
 * @version 1.0.0
 * @api 1.12.0
 * @description Way to open virtual custom inventories
 * @author iksaku
 */
 
namespace VirtualInventories{
    use pocketmine\command\Command;
    use pocketmine\command\CommandSender;
    use pocketmine\command\PluginIdentifiableCommand;
    use pocketmine\inventory\ChestInventory;
    use pocketmine\nbt\tag\Compound;
    use pocketmine\nbt\tag\Enum;
    use pocketmine\nbt\tag\Int;
    use pocketmine\nbt\tag\String;
    use pocketmine\Player;
    use pocketmine\plugin\PluginBase;
    use pocketmine\tile\Chest;
    use pocketmine\tile\Tile;

    class Loader extends PluginBase{
        public function onEnable(){
            $this->getServer()->getCommandMap()->register("VirtualInventories", new VirtualCommand($this));
            $this->virtualChest();
        }

        /** @var null|Chest */
        private $fakeChest = null;

        /**
         * @return Chest
         */
        private function fakeChestTile(){
            if($this->fakeChest === null){
                $this->fakeChest = new Chest($this->getServer()->getDefaultLevel()->getChunk(0 >> 4, 0>> 4), new Compound("VirtualChest", [
                    new Enum("Items", []),
                    new String("id", Tile::CHEST),
                    new Int("x", 0),
                    new Int("y", 0),
                    new Int("z", 0)
                ]));
            }
            return $this->fakeChest;
        }

        /** @var null|ChestInventory */
        private $virtualChest = null;

        /**
         * @return ChestInventory
         */
        public function virtualChest(){
            if($this->virtualChest === null){
                $this->virtualChest = new ChestInventory($this->fakeChestTile());
            }
            return $this->virtualChest;
        }
	}

    class VirtualCommand extends Command implements PluginIdentifiableCommand{
        /** @var Loader */
        private $plugin;

        /**
         * @param Loader $plugin
         */
        public function __construct(Loader $plugin){
            parent::__construct("virtual", "Opens a virtual chest", null, ["v", "chest"]);
            $this->plugin = $plugin;
        }

        /**
         * @return Loader
         */
        public function getPlugin(){
            return $this->plugin;
        }

        /**
         * @param CommandSender $sender
         * @param string $alias
         * @param array $args
         * @return bool
         */
        public function execute(CommandSender $sender, $alias, array $args){
            if(!$sender instanceof Player){
                return false;
            }
            $sender->addWindow($this->getPlugin()->virtualChest());
            return true;
        }
    }
}

Is this working? ;)