There have been a number of discussions of this on the Forums. Due to varying machine sizes, budgets and availability / access to materials, each user tends to create their own solution.
One consideration is simply minimizing vibration so as to avoid amplifying the machine noise --- one solution for this is a pad placed beneath the machine. The drawer liners made for tool chests are one solution: Could someone please help me to identify this? --- an alternative is an inexpensive yoga mat.
Forum discussions of actual enclosures:
- http://community.carbide3d.com/t/so3-xxl-mega-enclosure-and-dust-collection/1764 --- includes dimensions
- soundproofing enclosures?
- Re: MeanderBolt's Build Log
- Just Enlarged my Shapeoko with some UpGrades
- Re: Zerblatt's build
- sound enclosure /lights/vacuum and dremel control --- build log album on Facebook.
- Edward's S2 Enclosure
- DIY CNC Dust Enclosure --- forum post w/ B.O.M.
- Re: Shapeoko 2 #7353 is Operational --- enclosure for a stock Shapeoko 2 of 3/4" MDF
- Shapeoko 3 enclosure thoughts --- includes detailed post on an earlier enclosure: http://www.shapeoko.com/forum/viewtopic.php?f=7&t=5277&p=46233#p38756
- tjshape's build log (Shapeoko 2 and a soundproof enclosure)
- CNC build: Enclosure --- includes price list
- Shapeoko 3 XXL + Enclosure Build 
- http://community.carbide3d.com/t/shapeoko-3-xxl-enclosure-build/8287 --- notable for using Basics Workbench parts
- Custom Enclosures for the Nomad and SO3
- Sketchup 3D Warehouse
- http://www.soundproofcow.com/Silent-Running-1.html 
- http://www.tectum.com/interior_walls.htm 
- the stepper motors themselves mostly run quietly! but it may be helpful to isolate them from the structure of the machine so as to reduce noise.
- Astrosyn Dampers (essentially rubber sandwiched between two brass plates)
- http://www.maytec.org/maycad_e.html --- CAD tool for designing w/ extrusions.
- Re: tjshape's build log (Shapeoko 2 and a soundproof enclosu --- baffle design for air out-port
- http://www.makealittlemore.co.uk/2016/02/10/diy-cnc-router-enclosure/ --- SO3 enclosure Based on a steel shelving unit, includes illumination and a myriad of nice features.
- https://imgur.com/a/QmdxL  
- http://www.dusty.tools/ --- http://community.carbide3d.com/t/cabinet-feedback-and-safety-question/9494 --- Faztek 1515 aluminum extrusion
3D printer discussion:
- airflow and router longevity: http://community.carbide3d.com/t/enclosures-for-the-shapeoko/11553
- heat/thermal expansion: http://community.carbide3d.com/t/building-an-enclosure-with-concerns-of-heat/2967