Fitting the MMU2 inside an IKEA Lack enclosure is a pain. 

You could somehow shoe the legs of your IKEA Lack, and raise it enough to fit the MMU2 underneath, and then you could have your enclosure, but you'd have no easy way to access and operate the MMU2. 

Opening a window is much better. The design is just the trimmings of the window, so your IKEA Lack still looks nice.

Whenever I need a full enclosure, I just cover the window I opened.