Think about the sound reflections.
Many times it is a matter of figuring *where* to put the mic as opposed to treating a room or building a booth.

Early reflections = trouble a lot of the time. A square room offers a lot of these early reflections as sound bounces around off four walls right back into the mic creating a 'boxy' sound.

Sometimes I find puting a vocalist in a long hallway with a large open end works well. The direct signal hits the mic and everything else travels down the hall and dissipates into a large room without any way to return to the mic.. just a three foot wide hall with an open end has worked surprisingly well at times.

Or you can build a booth. I've done that with good results also. It's those early reflections that seem to cause the most trouble in my experience.

I do not work here, but the benefits are still awesome
Make your sound your own!