Give Roy a call at Green Mountain and see what he thinks about the use of the Europa in that situation. It is front ported and may or may not work. Also the Reynaud Twin is a front ported transmission line. Also as a previous 5.O owner, I don't think they will work in this situation. Another though harder to find option may be the Snell bookshelves as some of them are sealed enclosures.
Jack
Jack