Well if they are anything like the rest of the klipsch in-walls from klipsch, they were designed for an open baffle. Trey who no longer works there has told me numerous times that some of the RP series in walls and in ceilings were designed for open baffle and would require something like a 3.0 cuft internal enclosure to remain the same sounding. anything smaller would severely harm the extension of them. Like i said, that was for the non thx speakers, but i can't imagine that being any different.
Are you just wanting to enclose them to try and contain some sound?