400kg - boy that's one hell of a soundbooth, and I guess a tidy sum. (maybe it's the Fred Dibnah wrought iron custom shop version
)
Are you sure you need one? I've been in a few studios in my time and never seen a vocal booth, if you are up for building such a booth in one of your bedrooms, it might be more worthwhile to pay less and get the room properly tuned acoustically, if that is why you are doing it, as for the wailing affecting those downstairs, there are a few options here....
Hope this helps.
Rich