Shorten the tubing inside the enclosure. Reduce the bend angles as much as possible. The vacuum performance will go up markedly.
I'd say it cut the noise level by half which is a huge improvement.
Half? That's only 3 dB (3dB = 1.995X). Measure, I bet you're much better than that!
For reference, the Nomad with it original enclosure runs at 61-65dB with the door closed. I run at 52-54dB, measured 2 feet from the enclosure front, door closed.
Find a smartphone app and measure.
Get a larger vacuum host for the inside to help with suction and sound level when it's running
I have a 2.5" connector right on the enclosure. The 50 mm tubing attaches directly. Inside the enclosure I have an adapter for the tubing.
Where is the air inlet (air out has to equal air in)? The cabling inlet? Can you please add a picture to your posting showing them? In my enclosure they are one and the same.
Did you caulk yours? I have seals that hold the sides. Regardless, I caulked them to ensure no whistling. Overkill... but good insurance.
What's the bottom design and material?
Anything special keeping the door closed?
Buy a larger underlayment that is meant for keeping vibration to a minimum. The current one is fine, but not meant for this application.
I have feet under mine so I have 4 independent decouplers. Sheet decoupling is fine. I usually go with 0.125" or 0.25" Neoprene.
Potentially add a camera
The camera is nice as one can watch from "anywhere". I went with camera only - no window - to obtain the maximum possible sound suppression (the door is covered just like the top and sides).
My camera is steerable. I sometimes move it around but it's mostly overkill. My camera had infrared illuminators; they work so well there are times that I forgot to turn the LED on!
I'm milling items that require high precision so I can't chance jeopardizing that.
My understanding is the Nomad is tested for precision and accuracy without sides. I'm sure the sides help in some circumstance... but not by much. Keeping them on certainly doesn't hurt.
I, like you, allowed plenty of extra space... but you're is considerably larger than mine. If I did mine again I would add 2" more on the sides; plenty of depth.
I left the back on. That made structure sense and it's a pain to remove anyway.
Since my original posting I added 6 inch tall poly "sides". They add some stiffness but primarily they keep any large stray swarf from escaping the sides. Easier cleanup.
One of the reasons I left the sides off is that they tend to channel they sound out the front... the side sound proofing doesn't get to do it's job (very much).
Add some strip lights so I can see what's going on
I love my LED bars. I recommend that lighting be done such that it doesn't radiate directly at you. Consider a dimmer... they can get pretty bright.
My friend Tim went with two LED spot lights. YMMV.
Do take time to think on the color temperature that works for you. I prefer clean white; others like the softer, more yellow choice. YMMV.
Get a small touch screen and dedicate it to Carbide Motion so I don't have to keep one of my computers tethered to the Nomad. I would automatically have the files at the ready thanks to Dropbox.
There are postings about low cost Windows tablets (< $200) being used quite satisfactorily. It doesn't take much to run CM.
or I want to add things to the inside
Leave any electronics outside. The small particles tend to accumulate on them. @Tshulthise left them outside on his SO3 XXL enclosure.
Nice dust head!