Sounds like the early ideas are best. You might also look around for someone who works with recording studios, if you have any in your area.
You can probably make this work, though it might take a lot of sound board & carpet. If you've got a lot of concrete and flat walls, baffles might not be enough.
Another issue will be the delay from front to back. If the surround delay in your box (whatever it is) isn't up to the job, you might need to add this component to your sound system and split the back surrounds from the front ones.
Decent stereo might also be out of the question, given the narrow room. Front/surround might be best, since people in the back probably wouldn't hear the stage sides, if you had them. Still, if you get the acoustics under control, even this can sound pretty good.