The solid box idea might be OK on carpet. A wood floor may resonate particularly since the box would form an acoustically resonant cavity. At my friend's, I mounted little rubbers under the M5 and the stand's legs in an attempt to de-couple from the floor. Works well!

The M60 may be a better bet but I can't say whether it would acoustically disappear as well as the M5.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated