After building a CNC machine I quickly realized I needed something to enclose the machine for both noise mitigation and dust control. So I built a pretty simple OSB box. The sound has been greatly reduced as has the dust. The project required two 4'x8' OSB sheets, about six 2"x4"s, and a 24" x 28" plexiglass pane. Regular square drive screws were used to attach all sides.
A normal conversation can now be had while watching the machine do its thing. A very worthwhile project for loud CNCs.