Only building something to keep out most of the exterior noise will do. For making a better sounding recording space you can just use duvets. But if exterior noise is present then you are wasting your time.
The deal with spot solutions is you have to map your room to know what to put in it. Put earplugs on the walll????What about sound panels made with MLV like this:
"as efficient as a solid core door" - at $215 per panel, just buy the doors!What about sound panels made with MLV like this: