My Taz5 sits on a hardwood butcher block work table that weighs a ton. The printer sits directly on the wood surface and it’s plenty stiff.
The Overkill Enclosure I’m building is made out of T Channel Aluminum and Plexiglas. The Enclosure also sits on the Table (around the edges) and has an open bottom. My inspiration to use Aluminum and Plexiglas came from here - http://imgur.com/a/O3H5r
Designing and building it was more fun than my first Erector Set!
I wouldn’t attach my printer to the enclosure. I think it would be very noisy. The enclosure would resonate with the song of the stepper motors. One benefit of the enclosure is to dampen the noise. I used rubber gaskets to secure the Plexiglas panels in the Aluminum Frame, but still think it would amplify the noise in my setup. It would also be harder to access the printer if you want to do any fixing or modifications. You would have to detach the Printer from the Enclosure before you could work on it.
The Electronics Box definitely needs to either be outside of the enclosure or have access to fresh, cool air. I designed and printed a small duct that penetrates through the Plexiglas and mates to the Electronics Box. On the Taz5, both the intake and exhaust happen on this side so all is well. You can see my Duct project here - https://forum.lulzbot.com/t/enclosure-duct/4742/1
I should be able to share my completed project in a week or so.