Galaxie make a couple of enclosed trailers. I don't have any in stock at the moment, but are easy enough to get.

http://www.elmcityhobbies.com/shop/index.php?main_page=product_info&cPath=4_183&products_id=2699

http://www.elmcityhobbies.com/shop/index.php?main_page=product_info&cPath=4_183&products_id=1739